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Design >> Analog Design >> subthreshold opamp
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Message started by newic on May 26^th, 2013, 4:34am

Title: subthreshold opamp
Post by newic on May 26^th, 2013, 4:34am

If an op-amp operates in subthreshold region, will it have better offset as compared to a opamp operate in saturation region? Assuming the subthreshold opamp has larger W/L for a given current.

I have doubt because each transistors conduct very small current in subthreshold region.

Title: Re: subthreshold opamp
Post by raja.cedt on May 26^th, 2013, 4:40am

hello,
mismatch in this region is much higher compared to inversion sat.

Title: Re: subthreshold opamp
Post by newic on May 26^th, 2013, 8:02am

May I know the reason?

Title: Re: subthreshold opamp
Post by boe on May 28^th, 2013, 5:31am

newic,
Delta Vt has greater effect in sub-threshold op point.
- B O E

Title: Re: subthreshold opamp
Post by Vladislav D on May 28^th, 2013, 8:46am

newic wrote on May 26^th, 2013, 4:34am:

You cannot say so. Depends on design. For example, a differential pair has more gain in a weak inversion. So, input referred offset is smaller.

Title: Re: subthreshold opamp
Post by Lex on Jun 3^rd, 2013, 2:21am

Suppose you lower the current to operate in subthreshold, the mismatch becomes higher. This is due to the effect that mismatch goes rapidly up when operating in subthreshold because of the shallow channel and effectively losing some gain as well.

I can't tell you in case the transistor area is constant and W/L is the variable. You'd have to simulate. But please bear in mind that mismatch is not that accurately modeled in subthreshold generally.

Looking forward to your sim results.

Title: Re: subthreshold opamp
Post by newic on Jun 4^th, 2013, 5:35am

the monte carlo results (1000 runs) is much better in subthreshold region, where Vgs<Vt for a fixed current as compared to saturation where its W/L is smaller to meet Vgs>Vt+xx.

doubt on the results because of the modeling accuracy.

How could i confirm transistors are operating in subthreshold region? Some forums said that the probe Vth (LV9) is not a true threshold voltage, the actual Vth is much smaller than LV9.
something related to LV142???

Title: Re: subthreshold opamp
Post by AnalogDE on Jun 4^th, 2013, 1:52pm

You want your mirror devices to be operating in strong-inversion, high overdrive. This reduces current mismatch between matched mirror devices.

You want to maximize your input pair gm -> high W/L -> low overdrive / subthreshold operation. You need some confidence of how well the model fits for low overdrive here. High W/L ratio -> less voltage offset and higher gain to minimize input referred offsets coming from the other matched devices in your opamp.

Title: Re: subthreshold opamp
Post by newic on Jul 26^th, 2013, 10:31pm

so it is a matter of model accuracy for sub-threshold.
So the Vth LV9 can be used for deep submicron?

Title: Re: subthreshold opamp
Post by Kevin Aylward on Jul 27^th, 2013, 1:42am

Matching is worse in subthreshold, although model accuracy is also a problem. Set up test current mirror and differential pair simulation, although the results are basically the same. Put a 10mv dc source in the gate or source of one of the transistors and see what the ratio difference in currents is when the current is swept. Spot numbers for a standard 220n/180n process gets ~25% mismatch for Vgst=-150mv to 8% mismatch for Vgst=+100 mv, falling to 2% mismatch at Vgst=+0.5V. Why? Well, 10 mv added on to 1/2V is a lot less than 10 mv added on to 100 mv, so mismatch goes approximately as Verror/Vgst, until it gets down to Vgst=0, where the approximation collapses, as the idealised approximation says that below threshold, current is zero! The exponential nature in subthreshold results in approximately a constant error ratio over much of the range. For general cmos design, keep Vgst above 100mv worse case, especially for current mirrors. Various performance parameters fall off below that value, although, sometimes 20 mv or so above zero Vgst may be ok. Noting that increasing W as an attempt to increase gm results in ever diminishing returns as Vgst gets below around 50mv, with a corresponding larger proportionate penalty in capacitance. Subthreshold is used because preventing subthreshold at very low currents requires very large gate lengths, which slow the system down even more that operating in subthreshold itself does. If the option is available, a bipolar is usually better for low currents, its gm is always higher, at any current.

Title: Re: subthreshold opamp
Post by RobG on Aug 11^th, 2013, 11:10am

I agree with AnalogDE, but let me expand. In general, lower bias currents result in worse mismatch because the devices have to be smaller. However, for any given current the opamp topology with the least offset will have the diff pair being in subthreshold and the mirrors laving as large of Vgs-Vt as you can get away with (at some point you'll run out of headroom or mobility mismatch will dominate). In other words, maximize the diff pair g_m, and minimize the mirror g_m. Subthreshold operation maximizes the diff pair gm for a given current so it is best for the diff pair.

If you do the math the offset is something like A_VT(DIFF)/sqrt(W_DIFF*L_DIFF)+ (g_mDIFF/g_mMirror)*A_VT(MIRROR)/sqrt(W_MIRROR*L_MIRROR),
where A_VT is the so-called Pelgrom coefficient and is the mismatch per unit area. Note the ratio of g_ms in the second term. The equations break down for shorter processes, but the concept is the same.

All that said, operating your diff pair in subthreshold usually limits your bandwidth. For my circuits I usually use Vgs-Vt = 150mV which is on the edge of subthreshold. Below that give diminishing returns and just starts killing your stability and bandwidth.