I have mentioned about this in my post 34. That most op-amp simulation models (model statements) incorrectly model the power-supply current.
And this is why this circuit will never work in simulation program. In real life the circuit might work, but I would add emitter degeneration resistors into current mirror.
Here you can see the proof.
If I use LT1363 as a op amp LTspice gives the correct result. But if I change the op amp to TL072, I don't get correct result.
By changing op amps of current mirrors to LM358 and adding (250 ohm) resistors to emitters of transistors, output of current mirrors works right. With 1Vdc on both channels I got 0.1v. by applying -1Vdc I got -0.097 v. At the output of buffers I got almost the same value.
But when upper channel is 0Vdc and lower is 1Vdc (between 10k resistors is still grounded ), Output of top buffer is 1.63mv and the lower buffer is 0.1v. Also, the last output is 0.99v.
As another note, i had to use the LM358 in this simulation because the LT model for the first op amp i tried DID NOT draw any current through the power supply pins! None at all, so that illustrates clearly that not all models are suitable for simulation of this kind of circuit. The LM358 model seems to work fine though, and you can see the 1 ohm resistors used i the power pins to sense current just to check to make sure it works right.
*//////////////////////////////////////////////////////////////////////
* (C) National Semiconductor, Inc.
* Models developed and under copyright by:
* National Semiconductor, Inc.
*/////////////////////////////////////////////////////////////////////
* Legal Notice: This material is intended for free software support.
* The file may be copied, and distributed; however, reselling the
* material is illegal
*////////////////////////////////////////////////////////////////////
* For ordering or technical information on these models, contact:
* National Semiconductor's Customer Response Center
* 7:00 A.M.--7:00 P.M. U.S. Central Time
* (800) 272-9959
* For Applications support, contact the Internet address:
* amps-apps@galaxy.nsc.com
*//////////////////////////////////////////////////////////
*LM358 DUAL OPERATIONAL AMPLIFIER MACRO-MODEL
*//////////////////////////////////////////////////////////
*
* connections:
* non-inverting input 1
* inverting input 2
* positive power supply 3
* negative power supply 4
* output 5
*
.SUBCKT LM358 1 2 3 4 5
*
*Features:
*Eliminates need for dual supplies
*Large DC voltage gain = 100dB
*High bandwidth = 1MHz
*Low input offset voltage = 2mV
*Wide supply range = +-1.5V to +-16V
*
*NOTE: Model is for single device only and simulated
* supply current is 1/2 of total device current.
* Output crossover distortion with dual supplies
* is not modeled.
*
****************INPUT STAGE**************
*
IOS 2 1 5N
*^Input offset current
R1 1 33 500K
R2 33 2 500K
I1 3 44 100U
R3 55 4 517
R4 6 4 517
Q1 55 2 44 QX
Q2 6 7 44 QX
*Fp2=1.2 MHz
C4 55 6 128.27P
*
***********COMMON MODE EFFECT***********
*
I2 3 4 75U
*^Quiescent supply current
EOS 7 1 POLY(1) 16 49 2E-3 1
*Input offset voltage.^
R8 3 49 60K
R9 49 4 60K
*
*********OUTPUT VOLTAGE LIMITING********
V2 3 8 1.63
D1 9 8 DX
D2 10 9 DX
V3 10 4 .635
*
**************SECOND STAGE**************
*
EH 3 98 3 49 1
G1 98 9 POLY(1) 55 6 0 9.8772E-4 0 .3459
*Fp1=7.86 Hz
R5 98 9 101.2433MEG
C3 98 9 200P
*
***************POLE STAGE***************
*
*Fp=2 MHz
G3 98 15 9 49 1E-6
R12 98 15 1MEG
C5 98 15 7.9577E-14
*
*********COMMON-MODE ZERO STAGE*********
*
*Fpcm=10 KHz
G4 98 16 33 49 5.6234E-8
L2 98 17 15.9M
R13 17 16 1K
*
**************OUTPUT STAGE**************
*
F6 4 3 POLY(1) V6 300U 1
E1 3 23 3 15 1
R16 24 23 17.5
D5 26 24 DX
V6 26 22 .63V
R17 23 25 17.5
D6 25 27 DX
V7 22 27 .63V
V5 22 21 0.27V
D4 21 15 DX
V4 20 22 0.27V
D3 15 20 DX
L3 22 5 500P
RL3 22 5 100K
*
***************MODELS USED**************
*
.MODEL DX D(IS=1E-15)
.MODEL QX PNP(BF=1.111E3)
*
.ENDS
*
What do you mean by the first stage? you mean opamps and the current mirrors connected to them? why do they have the unity gain? I thought each of op amp with current mirrors and 1k resistor on the output of them has the gain of 0.1.There are a couple ways to change the gain.
Part 1:
If you look at the first stage of each section, you see a gain of 1 buffer. You know how that works. If you input 1v you get 1v out. If you input 2v you get 2v out, etc.
Part 2:
Next, there are the two 10k resistors on the output of the input buffers. With 1v in (and 1v out) we get 1/10000 amps through the 10k With 2v in we get 2/10000 amps through the 10k.
Doing all three of these things would increase the gain by a factor of 1000, at least theoretically. You'd have to investigate the best method here. For example, raising the gain of the output stage means imposing more of a limit on the upper frequency range for the circuit.
Thank you so much for your good explanation. I have one question.Why does raising the gain of output stage impose more limit on upper frequency? I hope in real life when I breadboard it I get this result, because op amps that I have are TL081. while in simulator I used LM358. I don't know why do you think LM358 is better than TL081? You said they have different model statement in LTspice because of that you choose LM358. but I want to know these differences
may affect the result that I'm supposed to get in real life or not?
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