# Simulation of implemented amplifier in LTSpice

#### robi10101298

Joined May 7, 2020
23
Hello, I need to simulate the following circuit in LTSpice. I made the scheme but I don't know what value to choose for R2 and Cinf in order to have theoretical gain Av=1500. The following constraints are imposed: Vcc=10v; VCM=1.5V; vg it's a sine wave with a frequency of 1Khz and R1=15kOhms. I need to simulate the circuit(transient, AC sweep), choosing an appropriate amplitude for vg in order to minimize output signal distortion(I put 100 uV but idk if it's correct..), and to specify a method for decreasing the voltage gain (without changing R1, R2, VCC or VCM). Implement the change and re-simulate the circuit in order to demonstrate the effect.

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#### Alec_t

Joined Sep 17, 2013
11,808
Is this a school/college assignment?

#### robi10101298

Joined May 7, 2020
23
Is this a school/college assignment?
As I mentioned, no, it's a project in where I learn how to simulate different behaviours of this circuit .

#### ronsimpson

Joined Oct 7, 2019
1,125
specify a method for decreasing the voltage gain
R3, R4, negative feedback is one method.

#### Audioguru again

Joined Oct 21, 2019
2,410
1) Your parts are so far apart on the schematic that the text is tiny.
2) Your "Tran" timebase stops in 3 seconds and starts at zero seconds. Then it shows 333 cycles of 1kHz for every inch. Make it stop in 100ms and start in 93ms with a maximum timestep of 1ms.
3) Your sinewave generator has infinite series resistance, make it 1 (ohm).
4) Your output capacitor is 1 Farad(!) so it takes a week to charge. Use a 0.1uF output capacitor that passes 1kHz perfectly and passes 150Hz into the 15k load very well.

Now you will see that the output transistors collectors will be at +9.9975V which is too high because the amplifier has no negative feedback causing it to have almost infinite DC gain. A very small DC offset voltage causes its gain to amplify it.
I removed Vcm and replaced it with negative feedback from the collectors of the output transistors and increased Vcm1 to +5V then the amplifier works fine.

#### robi10101298

Joined May 7, 2020
23

#### robi10101298

Joined May 7, 2020
23
1) Your parts are so far apart on the schematic that the text is tiny.
2) Your "Tran" timebase stops in 3 seconds and starts at zero seconds. Then it shows 333 cycles of 1kHz for every inch. Make it stop in 100ms and start in 93ms with a maximum timestep of 1ms.
3) Your sinewave generator has infinite series resistance, make it 1 (ohm).
4) Your output capacitor is 1 Farad(!) so it takes a week to charge. Use a 0.1uF output capacitor that passes 1kHz perfectly and passes 150Hz into the 15k load very well.

Now you will see that the output transistors collectors will be at +9.9975V which is too high because the amplifier has no negative feedback causing it to have almost infinite DC gain. A very small DC offset voltage causes its gain to amplify it.
I removed Vcm and replaced it with negative feedback from the collectors of the output transistors and increased Vcm1 to +5V then the amplifier works fine.
I can;t modify the parameters for my scheme, Vcm,Vcm1, Vcc and R1 should be fixed. Also vg it's a sine wave with a frequency of 1Khz , amplitude can be anything.

#### Audioguru again

Joined Oct 21, 2019
2,410
This is my simulation:

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#### robi10101298

Joined May 7, 2020
23

#### crutschow

Joined Mar 14, 2008
26,053
Why is this analog circuit posted in the Digital Design forum?

#### Audioguru again

Joined Oct 21, 2019
2,410
You simulated Vg, no?
My simulation shows Vg as 1.5mv peak.
I think having the gain of 1500 times is foolish because of input offset voltage , noise and that the circuit is biased wrongly causing clipping of the bottom of the waveform.

#### robi10101298

Joined May 7, 2020
23
My simulation shows Vg as 1.5mv peak.
I think having the gain of 1500 times is foolish because of input offset voltage , noise and that the circuit is biased wrongly causing clipping of the bottom of the waveform.
Hello, yes, I got almost 1000 gain in simulations, so you're right...

#### Bordodynov

Joined May 20, 2015
2,744
For more amplification, you have to set more current through transistors.

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#### Bordodynov

Joined May 20, 2015
2,744
For more amplification, you have to set more current through transistors.

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#### atferrari

Joined Jan 6, 2004
4,172

#### Bordodynov

Joined May 20, 2015
2,744
Hola Alex, something seems wrong with your attachment.
But you see an electronic circuit? I don't know what happened. I just removed one picture and replaced it with another.

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#### test4

Joined Mar 14, 2019
7
robi10101298, in your circuit, the Q8 base hangs in the air. See SPICE Error Log.

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#### atferrari

Joined Jan 6, 2004
4,172
In post #13, where it says "View attachment 208249" that link gives error 404.
The next post below shows OK.

/edit
I believe you made a second post (14).
I suggest you could delete #13.
Edit/

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