Does op amp GBP (gain bandwidth product) affect DC gain? (non-inverting buffer) - Page 1
Forum section: electronics / beginners

author: cape zoloh
author_rank: Contributor
timestamp: July 04, 2020, 02:18:48 pm
content: I usually foul around with the Falstad Circuit Simulator, and I noticed that GBP affects the DC gain of an op amp.Is this true, or is it a bug in the code of the software?The circuit explains it all:https://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+10.20027730826997+57+5+50%0Av+-32+304+-32+224+0+0+40+1+0+0+0.5%0Ag+-32+304+-32+320+0%0AO+288+208+352+208+0%0Aw+-32+224+96+224+0%0Aw+96+192+96+144+0%0Aw+96+144+288+144+0%0Aw+288+144+288+208+0%0Ap+288+208+288+304+1+0%0Ag+288+304+288+320+0%0Aa+96+208+288+208+8+15+-15+1000000+0.999990000099999+1+100000%0Ap+96+224+96+304+1+0%0Ag+96+304+96+320+0%0Ax+-13+269+93+272+4+12+%3C---Input%5Csvoltage:%0Ax+188+261+282+264+4+12+Output%5Csvoltage:%0Ax+369+262+564+265+4+12+(10mV%5Cslower%5Csthan%5CsInput%5Csvoltage)%0Ax+185+162+274+165+4+12+%22Ideal%22%5Csop%5Csamp%0Ax+190+179+270+182+4+12+GBP%5Cq100KHz%0Ao+0+64+0+4098+5+0.00009765625+0+2+0+3+Input%0Ao+2+64+0+4098+5+0.00009765625+1+1+Output%0A

author: magic
author_rank: Super Contributor
timestamp: July 04, 2020, 02:26:02 pm
content: It's some error, maybe wrong translation?To me it shows up as open loop gain being 100k, which totally makes sense.I don't even know how to set GBW in this simulator.

author: TimFox
author_rank: Super Contributor
timestamp: July 04, 2020, 02:28:18 pm
content: For a simple op-amp model, the GBW is the frequency at which the gain falls to unity, and the DC gain of the op amp is independent of the GBW. As you increase from 0 to a frequency given by (GBW)/(DC gain), this model has a high gain, independent of frequency, and then the open-loop gain falls off at 6 dB/octave until the frequency hits GBW. (This refers to the simplified frequency response, piecewise linear in dB vs. log f)The frequency dependence of gain will be different for op amps that are not unity-gain stable, for example.

author: David Hess
author_rank: Super Contributor
timestamp: July 04, 2020, 11:35:45 pm
content: In real operational amplifiers there are a couple of ways gain-bandwidth product affects DC gain. The one which likely matters in this case is that the transconductance of the first stage might be lowered to improve stability which also lowers DC gain.

author: magic
author_rank: Super Contributor
timestamp: July 05, 2020, 06:13:38 am
content: Quote from: David Hess on July 04, 2020, 11:35:45 pmOnly because it also reduces maximum open loop gain at low frequencies and DC. Otherwise there would be no change to closed loop DC gain, regardless of changing high frequency bandwidth.

author: cape zoloh
author_rank: Contributor
timestamp: July 05, 2020, 07:26:08 am
content: Very interesting to hear about this. My thinking was that this is a pure speed thing.In the simulator you rightclick on the op amp symbol to change the bandwidth.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 05, 2020, 08:10:47 am
content: In the simulator the GBW should not effect the DC gain.When designing real world OPs, very high speed OPs tend to have less DC gain, as this helps to make them fast and stable. Very high DC gain OPs tend to use more internal stages, which can be a problem at the highest speed.However there can still be slow OPs with low DC gain and relatively fast (e.g. 100 MHz GBW) OPs with high DC gain.

author: magic
author_rank: Super Contributor
timestamp: July 05, 2020, 09:40:09 am
content: Quote from: cape zoloh on July 05, 2020, 07:26:08 amNo, it sets open loop gain. And it makes sense that OLG affects CLG.editMystery solved. Here's your 100kHz GBW opamp happily passing a 1GHz signal.Seriously, this optionis not GBW. Their opamp model doesn't even seem to have any bandwidth limit at all, I bet it's full open loop gain at all frequencies with zero propagation delay.