How to design a multistage discrete BJT amplifier

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ericgibbs

Joined Jan 29, 2010
18,848
Hi GF
That's pretty good, close to vc=8.5v.
Added a 100mVpk, 1kHz sine wave input signal to show the performance.


Added: what do you make the Voltage gain, use the vc output plot.

EEG57_ 836.png
 
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MrChips

Joined Oct 2, 2009
30,802
I am just a bit late to the party.

A classic amplifier design consists of three stages.
(1) preamp stage
(2) driver stage
(3) output stage

The purpose of the preamp (1) is to provide high input impedance and moderate gain.
The function of the output stage(3) is to provide power, current gain, and low output impedance.
The middle driver stage provides additional voltage gain, low matching input and output impedance to match the preamp and output stage, along with additional low and high pass filter to meet the required specifications.

In addition, there should be negative DC feedback from the output to the driver in order to improve linearity and provide temperature stability. The driver would be DC coupled to the output stage.

The specs call for an input of 200mV and gain of 50. Hence the amplifier has to provide ±10V at the output. You need to calculate the current and power into the load.
 

WBahn

Joined Mar 31, 2012
30,052
Think about how your sources are hooked up.

If you look at V2, the positive terminal is connected to your 0 V common (a.k.a., "ground") and the negative terminal is connected to your top rail. With the output voltage set to +15 V, that means that the top rail is 15 V LOWER than ground.

Now consider what it means to have a -15 V source (V3) connected with it's positive terminal connected to ground.

Are these the voltages you intended?
 

boostbuck

Joined Oct 5, 2017
514
If you search Google there are plenty of documents describing the process of designing an amplifier such as yours, including calculating the circuit elements.

There's no shortcut to understanding - you have to get your head around the principles involved.
 

Thread Starter

GF Oberholzer

Joined May 23, 2023
39
If you search Google there are plenty of documents describing the process of designing an amplifier such as yours, including calculating the circuit elements.

There's no shortcut to understanding - you have to get your head around the principles involved.
I have googled and found nothing that is why I came to this forum. So if you can send me a link I will greatly appreciate it and try to solve it on my own.
 

ericgibbs

Joined Jan 29, 2010
18,848
The specs call for an input of 200mV and gain of 50. Hence the amplifier has to provide ±10V at the output. You need to calculate the current and power into the load.
Hi C,
It is specified as 200mV peak to peak, for 50:1 gain, so +/-5V for Vout.
E
Clip:
The input signal shall be a 200 mV peak-to-peak sine wave
 

ericgibbs

Joined Jan 29, 2010
18,848
Hi GF,
How did you calculate R1 as 76.45K. ?
Try a value around 120K.
Post your calculations and simulation.
E

I assume you are using a BF=200.?
 

ericgibbs

Joined Jan 29, 2010
18,848
Hi GF,
I make the Q1, Ib approx 20uA, I will work through your calculations.

Would you try calculating R3 and R4 to set the gain for 10 at Vo1, on this test circuit.?

E
EG57_ 840.png
 
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