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https://designers-guide.org/forum/YaBB.pl Design >> RF Design >> highly linear current mirrors https://designers-guide.org/forum/YaBB.pl?num=1324771933 Message started by sandman on Dec 24th, 2011, 4:12pm |
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Title: highly linear current mirrors Post by sandman on Dec 24th, 2011, 4:12pm Hi, I'm trying to implement a highly linear current mirror as a transconductance stage of a passive current-mixer. The mixer is driven by a current source that drives current into the mirror input. The output current (from the mirror) is then driven into a load which the passive mixer then (voltage) switches into the load of the mixer (resistive, small). This is for a transmit chain. I've tried some basic topologies (e.g. source degeneration) which give me decent results for linearity (> +30dBm), but it burns up a lot of current and is noisy. Could anyone suggest a very linear (>30dBm) transconductance or a means to further linearise a current mirror and yet achieve <2nV/√Hz (<-160dBm/Hz) output noise floor with some small gain ? Or even any seminal papers or interesting designs/topologies ? I know this isn't all of the information about my design, but I would be happy to answer any questions and would welcome any suggestions ! cheers |
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Title: Re: highly linear current mirrors Post by raja.cedt on Dec 25th, 2011, 2:08am hello, even i was thinking of high linear opamp, degeneration would give linearity up to some extent. Coming to current mirror to get high linearity use vds matching by using feedback through some opamp and to get less noise make sure that your mirroring ratio is small. Thanks, raj. |
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Title: Re: highly linear current mirrors Post by loose-electron on Dec 25th, 2011, 4:03pm A current source, by design is high impedance. High impedance = high R equivalent for noise. Current source will always be noisy, due to being a current source! |
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Title: Re: highly linear current mirrors Post by raja.cedt on Dec 26th, 2011, 1:18am hello looseelectron, high impedance means higher voltage noise, but normally current sourse will be terminated by lower impedance with some kind of cmfb or some sort of feedback makes impedance lower. so i guess decreasing current source current noise will be help full. But if current current source terminated by high impedance what you are saying is correct. One more thing do you aware some kind of opamp linearization techniques apart from degeneration. Thanks, Raj. |
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Title: Re: highly linear current mirrors Post by RobG on Dec 26th, 2011, 7:54am sandman wrote on Dec 24th, 2011, 4:12pm:
For noise, I suggest starting out with the equations and substituting 2*I/(Vgs-Vt) for gm in your final equations. This will give you a physical feel for the trade-offs that you have. Also, you should be able to look at the simulation results and determine the contribution of each device in the circuit after doing your noise analysis. Current mirrors are pretty basic: the lower the gm, the lower the noise they produce. If you lower the gm by decreasing the width, the mirror will be faster. If you lower gm by increasing the length, the mirror will be slower. Physically, lowering gm means making Vgs-Vt as large as possible for a given current so a low noise current mirror will need lots of headroom. Alternatively, resistive degeneration accomplishes the same thing, and is even better for 1/f noise. Drop as much voltage as you can stand across the resistor. I do not know why this didn't work for you, but you must be misinterpreting the results or describing the circuit incorrectly to us. Furthermore, your current consumption should stay the same - I suspect you are doing something wrong as the current consumption increase should reduce the noise, assuming you kept the gain the same. The noise could also be generated before the mirror: garbage in, garbage out. I assume finite output impedance of the mirror is causing your linearity problem. Resistive degeneration will help as you found out, as will a cascode. The former will reduce noise,; the latter will add negligible noise for a given operating point, but the bigger impact of the cascode will be loss in headroom which will limit how much vgs-vt you can have. In other words, you will lose headroom by adding the cascode, forcing you to reduce vgs-vt of the mirror (or the IR drop if degenerated). The loss in headroom will force a design with higher noise. rg |
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Title: Re: highly linear current mirrors Post by loose-electron on Dec 26th, 2011, 12:48pm raja.cedt wrote on Dec 26th, 2011, 1:18am:
Raj, ok agreed when you include the termination. Linearization of op-amps? Um, when its in a feedback configuration, the general tool is to increase the open loop gain, and the BW, so you have sufficient gain at the frequency of interest. Generally, the above is the path to take. |
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Title: Re: highly linear current mirrors Post by aaron_do on Dec 27th, 2011, 7:20am Hi sandman, just wondering why do you have a current input in the first place? Is the entire transmit chain current-mode? regarding your question, you have two figures that need optimization: noise and linearity. So I guess you need to do some kind of study to see which type of current mirror offers the best trade-off. Alternatively, convert your current into a voltage using an op-amp in feedback. It can be a simple op-amp design to minimize noise. Aaron |
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Title: Re: highly linear current mirrors Post by raja.cedt on Dec 27th, 2011, 7:47am hello aaroon, i feel working with current mode is better from noise point of view, because it involves less circuitry. I didn't understand "some kind of study to see which type of current mirror offers the best trade-off", could you please explain better... Thanks, Raj. |
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Title: Re: highly linear current mirrors Post by aaron_do on Dec 30th, 2011, 2:46am Hi Raj, Quote:
I suppose that different current mirrors offer different noise and linearity tradeoffs. For example, a cascode current mirror may be more linear since under large signal conditions, it is less affected by the output voltage (total Vds). However, because more devices are involved, it is likely to be noisier than a simple current mirror. So the question becomes, for the same noise budget, which current mirror offers the highest linearity? There are of course other tradeoffs. My comment about possibly using an op-amp refers to the fact that a transconductance stage is needed at some point anyway. Unless you have a current-output DAC and a current-mode transmit chain. Not sure exactly how that would be implemented. regards, Aaron |
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Title: Re: highly linear current mirrors Post by raja.cedt on Dec 30th, 2011, 3:26am hello aaron_do, yes you are correct, there is some sort of trade off but a cascode will improve linearity a lot but it won't add too much of noise. Any how good reply regarding current mirror. Do you know any architecture which matches vds very accurately, i heard some architectures which matches Vds by op amp but they have both +ve and _ve feedback. Regarding current mode, what i feel is working with current mode gives better noise and less complicated circuits, because for any filter you can do multiplication, subtraction and all other operation with good current mirror where as in voltage mode you have to use opamps. Any comments are welcome. Thanks, raj. Thanks, Raj. |
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Title: Re: highly linear current mirrors Post by sandman on Dec 30th, 2011, 9:19am loose-electron wrote on Dec 25th, 2011, 4:03pm:
Hi loose-electron, I have a current-source (sink rather!) DAC at the input of my current-mirror (CM) amplifier. The input current noise of the CM amplifier is amplified by the Gm (including degeneration,) of the diode connected device and then the mirroring ratio (>1 assumed). The output current noise of this combination sees the parallel combination r_out (small signal) of the CM and the R_load, which is typically smaller than r_out, resulting in a smaller output voltage noise (than across r_out alone). I wonder then if what you say about the r_out still holds, taking the load into picture. |
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Title: Re: highly linear current mirrors Post by sandman on Dec 30th, 2011, 9:21am raja.cedt wrote on Dec 26th, 2011, 1:18am:
Raj, I agree with you. I didn't notice your comment before I replied to loose-electron's previous comment. The voltage noise is load dependent although it is imperative to keep the current noise as low as possible. |
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Title: Re: highly linear current mirrors Post by sandman on Dec 30th, 2011, 9:33am Hi Aaron, aaron_do wrote on Dec 27th, 2011, 7:20am:
I have current-'sink' DAC that drives modulated signal current into the CM. The CM amplifier drops the current across a load. The voltage across this is switched by a passive mixer. This is for a Tx. chain, so the mixers draw no current (theoretically). I set out to explore the noise equations for a CM amplifier by hand but this has be expected-ly slow. A couple of papers treat noise in CM's but only through simulation results. Interestingly, I haven't come across any papers or articles on the linearity of the various current-mirrors. Any suggestions ? I did consider using op-amps at the input of the mixer. But, I needed some gain control and an output noise lower than -160dBm/Hz. Some basic analysis ruled out an Op-Amp configuration where I tried to enhance linearity.. What kind of structure did you have in mind ? |
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Title: Re: highly linear current mirrors Post by raja.cedt on Dec 30th, 2011, 10:13am hello sandman, why don't you post some rough sketch of your complete ckt. I am not that good at mixers. But being a analog guy i can comment on CM and opamps. Coming to current mirrors what do you mean by linearity, i mean increase the input current and check at the o/p current plot these two and check the ratio and till what point of input current your gain drops till 1 db or some margin. Coming to current mirror if you keep on increasing current, Vov start increase and this intern effects the mirroring ration, if you are facing this problem i would have a cascode or some vds matching. Did you tried like this. One more thing whats the max input current of your current mirror? Till now i didn't find any papers on CM noise, could you please tell me those names... Thanks, Raj. |
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Title: Re: highly linear current mirrors Post by RobG on Dec 30th, 2011, 11:42am sandman wrote on Dec 30th, 2011, 9:33am:
Where is the hangup? The noise source should just be current sources in parallel with the devices in the mirror. The analysis is pretty straight forward from there. As I mentioned earlier, about all you can do to reduce thermal noise is increase the overdrive (i.e. Vgs-Vt). Raj - I don't think you want to use an opamp; it would be overkill, limit bandwidth, use extra power, add noise, etc. Apparently he can get the linearity with resistive degeneration so in the worst case he should be able to do far better with a cascode device. |
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Title: Re: highly linear current mirrors Post by raja.cedt on Dec 30th, 2011, 2:47pm hello robg, yaa, with opamp it kills BW, but it wont add much noise because noise of the opamp will be again cascode noise what i mean is it gives better results compared to all from matching point of view. Bottom line is resistive degeneration better. Thanks, raj. |
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Title: Re: highly linear current mirrors Post by sandman on Jan 2nd, 2012, 2:56am raja.cedt wrote on Dec 30th, 2011, 10:13am:
Hi Raj, I'm still working on sketching out an intelligent representation for my mixer, but it is just at I described in my messages above. If I can manage it, I will try to post a schematic sometime soon... With reference to the linearity, I simply measure the OIP3 for a two-tone signal at the inputs of a current mirror. This is simplistic but sufficient because I have a sufficient back-off from the compression point and am not concerned about linearity near the compression zone. I'm also not very worried about the mixer's linearity because all I have are a set of well-driven passive mixers in the resistive region. Increasing the Vov lessens the Vth mismatch and also the level of noise, as RobG pointed out above. Having a cascode reduces the impact of having a low λ, which should help with a more accurate mirror ratio, but I observed that this did not improve the linearity and only worsened it... which is why I'm trying to locate papers which treat the linearity of Current-Mirror Amplifiers. Any suggestions anyone? I wonder if there is a technical term for such RF CM amplification circuits ... ? Vds matching using an op-amp as a -ve feed-back error amplifier would only add more noise and I wonder if that will improve linearity ? As for papers on noise, I found an old one from 1975 on IEEE - Noise Characteristics of Current Mirror Sinks/Sources A. BILOTTI & E. MARIANI, but I'm sure there are better ones out there. cheers, |
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Title: Re: highly linear current mirrors Post by sandman on Jan 2nd, 2012, 3:07am RobG wrote on Dec 30th, 2011, 11:42am:
RobG, thanks for your comments. While the treatment of a simple current mirror is straight-forward, I'm using a couple of feedback methods at the input which add their own noise to the input (or output) - hence the added complexity in the details... I simulated a simple CM alongside a CM with output-stage cascode. The cascode had worse output linearity and the noise was somewhat increased than compared to resistive degeneration. From what I'm familiar with, non-linearity in the CM comes about due to mismatches in the mirror and the transistor non-linearity (non-linear capacitances, deviation from square law, other non-idealities which I'm yet to figure out!). 'Cascoding' the output only compensates for the poor λ. There's a lot more to linearity than just mismatches that you point out. What are your thoughts on this ? Source degeneration is one method which works, but that's because it reduces the gain but has a limit to the dynamic range that can be achieved with this method. My input DC voltage is fixed, so there probably isn't much I could do, to play with the Vgs-Vth than increasing drain current, so (resistive) degeneration seems to be the only solution I can think of. |
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Title: Re: highly linear current mirrors Post by raja.cedt on Jan 2nd, 2012, 5:14am hello sandman, I didn't understand why cascode doesn't improve the linearity, becaz you are saying that cascode improves o/p impedance means linearity got improved. Check your cascode is biased properly or not. Again using opamp for Vds also some kind of cascode only. you can check some noise analysis here http://www.av.it.pt/ruilaa/papers/2002/icecs2002_noise.pdf |
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Title: Re: highly linear current mirrors Post by RobG on Jan 2nd, 2012, 7:18am sandman wrote on Jan 2nd, 2012, 3:07am:
OK, based on what you said you are losing linearity from things like non-linear capacitance, non-square law etc. That is a hard problem for us to help with since we don't know your topology. Personally, I'm surprised those things affect the mirror accuracy but we all learn new things. I assume these bad effects happen at high frequency and large signal (transient) which always changes what we know. Try it with the tolerances increased for sure! As for the noise analysis, you should be able to just do the mirror effects as the noise sources should not influence one another. Increasing current should help with noise and it sounds like it may be your only option. It will reduce your output impedance which might affect other things. These are things that generally work. It is entirely possible that there is something special about your circuit that is negating some of these effects. It is hard to help more without seeing your actual circuit. When I see things like this I usually dive into the equations and try to figure out what is going on taking into account different effects, but I understand that might be too difficult in your case. Starting with ideal devices is also helpful, although these days the model is so complex that it is difficult to do. |
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Title: Re: highly linear current mirrors Post by aaron_do on Jan 2nd, 2012, 5:13pm Hi sandman, Did I understand correctly that your current mirror is operating at IF? In which case, nonlinear capacitance would not affect the IIPn. Quote:
Also, IIPn is a very sticky issue IMO. How simple was the current mirror you simulated? You will often find that the nonlinearity of one stage compensates the nonlinearity of another, especially if there isn't much gain seperating the stages (In some circuits, we deliberately add nonlinearity for such purposes). For a fair comparison, I guess you should keep the design as simple as possible. Lastly, have you tried simulating a CM with both input and ouput-stage cascode? If only the output stage has a cascode, then the drain voltage of the input stage would change with the input signal level resulting in some mismatch. cheers, Aaron |
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Title: Re: highly linear current mirrors Post by sandman on Jan 4th, 2012, 1:08am raja.cedt wrote on Jan 2nd, 2012, 5:14am:
Hi Raj, I simulated a an improved Wilson. The bias is dependent on the external DC. If this is correct, then the cascode improves the output impedance, but that only goes to reduce the mismatch. Mismatch could only be a subset of the non-linearities at IF or RF if it degraded the IP3 when compared to the simple CM. Do you have an understanding of the other non-linear mechanisms in current mirrors operated at IF or RF ? cheers, |
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Title: Re: highly linear current mirrors Post by sandman on Jan 4th, 2012, 1:20am aaron_do wrote on Jan 2nd, 2012, 5:13pm:
Hi Aaaron, I simulated both a simple CM and an improved Wilson CM (input and output cascode). The biasing there appears straightforward. I agree with the IP3 being a naive assumption about device linearity. Besides, I also have a 1Vpp differential signal at the input ! I'm also considering the compression point. The input signal bandwidth is 0MHz to 500MHz. If this is low frequency for the capacitances to come into effect and non-linear capacitances are a big part of non-linearity, then I wonder why the improved R_out (reduced mismatch) didn't improve the linearity... Any suggestions? cheers, |
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Title: Re: highly linear current mirrors Post by aaron_do on Jan 4th, 2012, 1:39am Hi sandman, I didn't realise this was such a wideband system. In that case, nonlinear capacitance may have some effect. How much effect will depend on the ratio of the reactive to resistive impedance at the nodes which are swinging. At the input side of the CM, nonlinear cap may not have much effect due to the low input impedance, but at the output side, nonlinear cap may be very important due to the high output resistance. One thing though, how are you checking the nonlinearity for such a wideband system? If you use IIP3, what are the two tone frequencies? I guess ultimately you are concerned with the spectrum mask requirements as well as EVM. Whatever it is it sounds like you've got quite a task ahead of you. The P1dB is mainly determined by when the devices hit the triode region. Maybe you just need to optimize your biasing? cheers, Aaron |
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Title: Re: highly linear current mirrors Post by loose-electron on Jan 4th, 2012, 6:55am Running 500MHz through a current mirror and expecting BW and linearity is probably not going to happen. As frequency goes up, more and more of that high impedance output current source is getting lost to parasitic capacitance. I would consider other options. |
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Title: Re: highly linear current mirrors Post by RobG on Jan 4th, 2012, 7:58am loose-electron wrote on Jan 4th, 2012, 6:55am:
I believe the output is terminated with an impedance much lower than the mirror output impedance, so mirror output impedance should have negligible effects. In fact, current mode should be faster since the voltage swings are small. That is what Gilbert's multipliers etc exploit. rg |
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Title: Re: highly linear current mirrors Post by RobG on Jan 4th, 2012, 8:50am This topic is getting beyond my comfort range, but if it is nonlinear capacitance then using the narrowest device possible and still maintaining headroom requirements for the mirror should work better than resistor degeneration. Remember, you have to keep your gate/vdd voltage the same for this to be a fair comparison. I suspect that there is more than one thing going on. I suggest replacing everything but the mirror with ideal devices and get the mirror working. You might even have to replace part of the mirror (e.g. the cascode devices if you use them). Vccs with the same gm can replace a mosfet. You may have to set the minimum and maximum currents and add tiny capacitances (0.1 fF) to keep the circuit from converging to unrealistic operating points. Also try it at a lower frequency. Keep idealizing things until you get something that works and then use that as a starting point. Over the years I've wasted many months trying to get a non-working circuit to work by just trying different things. Almost always I would have been better off starting from an ideal circuit and working my way towards the final implementation one mosfet at a time. rg rg |
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Title: Re: highly linear current mirrors Post by loose-electron on Jan 4th, 2012, 10:15am RobG wrote on Jan 4th, 2012, 7:58am:
Then its a common source amplifier structure, not a current mirror. No schematics = No clear definition = No good answers... |
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Title: Re: highly linear current mirrors Post by aaron_do on Jan 4th, 2012, 4:56pm Quote:
[/quote] In order to see whether nonlinear cap at the output can have an effect, he only needs to run an AC analysis (maybe in this case a PAC analysis if the mixer is included) to check the bandwidth at the output node. If it is much more than 500 MHz, then he needn't worry. However, I don't think it's clear that the terminating resistance is low. He mentioned the mirror is followed by a "current passive mixer", not current-commutating devices (i.e. Gilbert Cell). Therefore, the terminating resistance of the CM will depend on the termination of the passive mixer, as well as things like LO drive, switch size etc. Although "current passive mixer" implies the passive mixer is followed by a low impedance, that is kind of atypical at RF (common-gate amplifier maybe?), so a schematic would help. cheers, Aaron |
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