Designer’s Guide Community

ywguo

Community Fellow

Offline

Posts: 943
Shanghai, PRC

Why does the GBW of VFB amplifier depend on closed loop gain?
Mar 01^st, 2013, 6:46am

Hi Guys,

I get to know current feedback amplifier and its counterpart voltage feedback amplifier (VFB) in some application notes of Intersil, Texas Instruments.

It is said that current feedback amplifier has GBW that is relative independent of close loop gain, while the voltage feedback amplifier's GBW strongly depends on the close loop gain. For Eg. , OPA820 of TI, http://www.ti.com/lit/ds/symlink/opa820.pdf, has small signal bandwidth of 800 MHz when close loop gain equals to 1. But it decreases very fast when the close loop gain increases. Its gain bandwidth product when the close loop gain = 20 is around 200 MHz.

Why does the GBW decreases as the close loop gain increases? and why does it decreases so fast?

Best Regards,
Yawei

Back to top

IP Logged

buddypoor

Community Fellow

Offline

Posts: 529
Bremen, Germany

Re: Why does the GBW of VFB amplifier depend on closed loop gain?
Reply #1 - Mar 1^st, 2013, 7:14am

Hello Yawei,

at first - it is NOT the GBW that decreases but the open-loop gain.
And this gain decreases not "fast" but with a constant slope of -20 dB/dec.
This is due to the single-pole frequency compensation for the open-loop opamp gain.

More than that, it is a general rule for negative feedback systems that the bandwidth gets larger with rising feedback.
That is one of the classical consequences of negative feedback.
The BODE plot for the open-loop and the closed-loop gain response vs. frequency is a very good tool to visualize this effect.

Back to top

LvW (buddypoor: In memory of the great late Buddy Rich)

IP Logged

raja.cedt

Senior Fellow

Offline

Posts: 1516
Germany

Re: Why does the GBW of VFB amplifier depend on closed loop gain?
Reply #2 - Mar 1^st, 2013, 10:50am

hello Yawei,
looks like Question is so clear. For example in any voltage feedback amplifiers GBW is fixed for a design and gain and 3dB bandwidth can trade each other, where you have written on the contrary.

current feedback circuits are another class of circuits.

refer this doc http://cas.ee.ic.ac.uk/people/dario/files/E416/OA-30.pdf

Thanks,
Raj.

Back to top

IP Logged

ywguo

Community Fellow

Offline

Posts: 943
Shanghai, PRC

Re: Why does the GBW of VFB amplifier depend on closed loop gain?
Reply #3 - Mar 1^st, 2013, 7:53pm

Hi Raj.

Quote:

For example in any voltage feedback amplifiers GBW is fixed for a design and gain and 3dB bandwidth can trade each other, where you have written on the contrary.

The following picture is a part of OPA820 spec. OPA820 is a voltage feedback amplifier. If we assume GBW is fixed, gain and 3dB bandwidth can trade each other, how do I explain the value of small signal bandwidth?

It is 800 MHz if G=1, V_O=0.1V_PP, R_F=0Ω. So the GBW is 800 MHz. Right?

And it is 240 MHz if G=2, V_O=0.1V_PP. So GBW becomes 480 MHz.

Even GBW decreases to 280 MHz if G≥20.

I am reading some literature about current feedback amplifier and voltage feedback amplifier. The GBW is one issue that confused me. � Sad

Thanks very much. What do you think about it?
Yawei

Back to top

OPA820_bandwidth_spec.png

IP Logged

buddypoor

Community Fellow

Offline

Posts: 529
Bremen, Germany

Re: Why does the GBW of VFB amplifier depend on closed loop gain?
Reply #4 - Mar 2^nd, 2013, 1:51am

Hello Yawei,

for my opinion, the TI datasheet is very confusing.
I suggest not to believe everything in such data sheets. Remember: It is a paper that shall help to sell the device.
More than that, the sheet mixes typical with minimum and maximum figures.

The OPA820 is a unity-gain stable amplifier (confirmed by the open-loop gain and phase response curve).
Thus, the GBW is identical to the zero-crossing frequency (transit frequency) and is a CONSTANT parameter.

*I do not understand why a TYPICAL GBW is given for >20 dB gain with GBW=280MHz.
At the same time, another raw (right hand) tells us that it is a minimum value.

*The given values are NOT in accordance with the information given in the graphs.
When for G=1 the typical BW is 800 MHz - why at the same time it is only 240 MHz for G=2 ?
According to the graph ir should be, of course, approx. 400 MHz.

Summary: Don`t be confused. Many data sheets are inconsistent and cannot explain the real behaviour.
Suggestion: So some invetstigations by yourself and simulate different gain configurations.
Good luck.

Back to top

LvW (buddypoor: In memory of the great late Buddy Rich)

IP Logged

ywguo Community Fellow Offline Posts: 943 Shanghai, PRC	Re: Why does the GBW of VFB amplifier depend on closed loop gain? Reply #5 - Mar 2^nd, 2013, 7:36am Hi buddypoor, What you said makes me a bit happy. Probably you are right. A datasheet is a paper that shall help to sell a device. I have test bench that gives lower bandwidth, which is shown below.
Back to top	stb_and_ac_bench_001.png IP Logged

ywguo

Community Fellow

Offline

Posts: 943
Shanghai, PRC

Re: Why does the GBW of VFB amplifier depend on closed loop gain?
Reply #6 - Mar 2^nd, 2013, 7:43am

The loop gain (purple line) and close loop gain (red line) is shown below.

The loop gain decreases to 0 dB at 180 MHz, while the close loop gain is -7 dB at 180 MHz. The -3dB bandwidth for the close loop gain is around 40 MHz only.

So what can I do to increase the close loop gain?

Any comments are appreciated.
Yawei

Back to top

stb_loop_gain_and_ac_close_loop_gain.png

IP Logged

buddypoor

Community Fellow

Offline

Posts: 529
Bremen, Germany

Re: Why does the GBW of VFB amplifier depend on closed loop gain?
Reply #7 - Mar 2^nd, 2013, 7:58am

ywguo wrote on Mar 2^nd, 2013, 7:43am:

The loop gain (purple line) and close loop gain (red line) is shown below.
The loop gain decreases to 0 dB at 180 MHz, while the close loop gain is -7 dB at 180 MHz. The -3dB bandwidth for the close loop gain is around 40 MHz only.
So what can I do to increase the close loop gain?
Any comments are appreciated.
Yawei

At first, the gain you call "loop gain" is the open-loop gain of the opamp with a maximum of approx. Ao=80 dB.
This is important to know because the parameter called "loop gain" is something else. It is the gain of the open loop including the feedback path.

At second, you ask how to increase the closed-loop gain?
Answer: Modify feedback resistor ratio.

Back to top

LvW (buddypoor: In memory of the great late Buddy Rich)

IP Logged

ywguo Community Fellow Offline Posts: 943 Shanghai, PRC	Re: Why does the GBW of VFB amplifier depend on closed loop gain? Reply #8 - Mar 2^nd, 2013, 10:16pm Hi buddypoor, There is one instance iprobe in the loop. And I run stb analysis with that iprobe cell. I think this is the loop gain including that opamp cell and the feedback path. Best Regards, Yawei
Back to top	IP Logged

ywguo

Community Fellow

Offline

Posts: 943
Shanghai, PRC

Re: Why does the GBW of VFB amplifier depend on closed loop gain?
Reply #9 - Mar 2^nd, 2013, 11:36pm

Hi buddypoor,

I have another picture of the close loop gain and loop gain. The close loop gain (red line) does not drop so fast. It is kept above 5.7dB at the phase margin frequency, 180MHz.

The point is ac analysis was run at DC point. The input is 1V and output is 2V, while the former ac analysis was run at tran time = 5.04ns in the former picture. I will have one wave form in next reply.

Best Regards,
Yawei

Back to top

stb_loop_gain_and_ac_close_loop_gain_at_DC_point.png

IP Logged

ywguo

Community Fellow

Offline

Posts: 943
Shanghai, PRC

Re: Why does the GBW of VFB amplifier depend on closed loop gain?
Reply #10 - Mar 3^rd, 2013, 12:12am

In the reply #6, the ac analysis is run at 50.4ns of tran time. Please look at the transient waveform in the following. The blue line "out" is the opamp output.

Although the voltage of opamp output is near 2V at the ac analysis time. Probably it is much different from the state at DC point because the opamp output is going down very fast.

Back to top

tran_ac_time.png

IP Logged

buddypoor

Community Fellow

Offline

Posts: 529
Bremen, Germany

Re: Why does the GBW of VFB amplifier depend on closed loop gain?
Reply #11 - Mar 3^rd, 2013, 2:32am

ywguo wrote on Mar 2^nd, 2013, 10:16pm:

Hi buddypoor,

There is one instance iprobe in the loop. And I run stb analysis with that iprobe cell. I think this is the loop gain including that opamp cell and the feedback path.

Best Regards,
Yawei

How could I know?
Yawei, now you see how important it is to provide us with all information necessary to answer your questions.
Why don`t you show us a circuit diagram?

Question: I don`t understand the meaning of your sentence "In the reply #6, the ac analysis is run at 50.4ns of tran time".
An ac analysis is time independent! Can you explain?
And what is your problem now? Everything looks good and stable.

Back to top

LvW (buddypoor: In memory of the great late Buddy Rich)

IP Logged