# how can i make temperature constant amplifier with bjts?

#### songhan

Joined Jun 8, 2021
19
If you start with a BJT with a current gain of 100 and make the gain of the circuit itself 10, then the gain will be 'fairly' constant even with temperature even though the Beta of the transistor changes with temperature. In this case the gain of 10 is sometimes referred to as the 'forced" gain.
You also have the issue of base emitter diode drop which also changes with temperature and that could cause the bias point to drift and thus the output offset. The output offset is more important in DC amplifiers but you did not specify AC or DC for your circuit yet.

Keeping the gain low (for each stage) and providing negative feedback might get you there but it depends how good you need it to be. For exceptionally good stability you probably want to go with a differential amplifier which has some degree of automatic compensation for drift.
i dont think i need that much good precision of stability, i just wanted to know about normal ways that we use to compensate the temperature effect. I just learned bjt parts of electric circuit class and i wonder that is there any way that i can compensate temperature effect of bjt amplifier. Because there is nothing about compensating temperature effect of bjt circuits in the book.

#### songhan

Joined Jun 8, 2021
19
At what frequency? And how much gain do you need?
about 100. And i want to get constant gain from varing frequency. So contant gain on bode plot.

#### Ian0

Joined Aug 7, 2020
3,260
about 100. And i want to get constant gain from varing frequency. So contant gain on bode plot.
Lowest and highest frequency?

#### songhan

Joined Jun 8, 2021
19
Lowest and highest frequency?
well at the middle point of the bodeplot would be best. i want a plot looks like trapezoid.

#### songhan

Joined Jun 8, 2021
19

this is all i made untill now. still unsure how to give negative feedback on it..

#### Ian0

Joined Aug 7, 2020
3,260
Try this: The two transistors whose part numbers overlap are BCM857C.
The open-loop gain will reduce by about 50% at -55°C according to NXP's datasheets, but that should make precious little difference to the overall amplifier.

#### songhan

Joined Jun 8, 2021
19
may ia
Try this: The two transistors whose part numbers overlap are BCM857C.
The open-loop gain will reduce by about 50% at -55°C according to NXP's datasheets, but that should make precious little difference to the overall amplifier.
View attachment 240779
um may i ask which part is the output part?

#### Ian0

Joined Aug 7, 2020
3,260
may ia

um may i ask which part is the output part?
Collector of Q5

#### BobTPH

Joined Jun 5, 2013
3,679
Ian0,

This is homework help.

Bob

#### songhan

Joined Jun 8, 2021
19
Collector of Q5
Ok thanks since you gave me example, i will try to understand how it works. it's my project i will just refer your work. i should study more how your circuit works.. I don't understand how your circuit works.
Anyway thanks for your help! it helped a lot

#### Ian0

Joined Aug 7, 2020
3,260
Ian0,

This is homework help.

Bob
Thanks for the reminder! It's a stock circuit (the innards of an op-amp) and we've already said "op-amp". The challenge now is to explain how it works, and why the gain remains stable over temperature!
I'd recommend any of the texts on audio design such as Douglas Self and J. Linsley-Hood.

#### songhan

Joined Jun 8, 2021
19
Ian0,

This is homework help.

Bob
i sware not goona use it directly on my project.
i will just refer it how this compensate the temperature effect

#### atferrari

Joined Jan 6, 2004
4,428
The OP seems to be revealing the required specs (creating them??) as the thread progresses.
Why not to transcribe verbatim the description of what he intends to do?
Starting with the inconsistent title.
Constant temperature? In outer space, maybe and I think, for not so long.

#### Ian0

Joined Aug 7, 2020
3,260
I thought it might be a satellite: 90°C when it's between the earth and the sun and -50°C when it's round the back.

#### Audioguru again

Joined Oct 21, 2019
3,542
You must learn about transistors to understand how the circuit works.
It has a gain of 1 like a piece of wire and an output resistance that is fairly high.

#### songhan

Joined Jun 8, 2021
19
You must learn about transistors to understand how the circuit works.
It has a gain of 1 like a piece of wire and an output resistance that is fairly high.
technically he teached it. But the problem is he taught only basics about bjts and gave such burdensome project like this...

#### Ian0

Joined Aug 7, 2020
3,260
I suggest that you read John Linsley-Hood The Art of Linear Electronics ISBN 0 7506 0868 4 which has less of the transistor theory, and more of the circuit theory. There is a very good section on design of amplifiers using bipolar transistors.
My circuit came out of one of his other books John Linsley-Hood Valve and Transistor Audio Amplifiers ISBN 0 7506 3356 5.

Now you can make a turntable preamp that will work down to temperatures where the grease in the turntable bearings has set completely solid, and the stylus compliance has gone completely stiff, and up to temperatures where the vinyl starts to melt!

#### DickCappels

Joined Aug 21, 2008
7,733
You must learn about transistors to understand how the circuit works.
It has a gain of 1 like a piece of wire and an output resistance that is fairly high.

In my first electronics class all we learned about were vacuum tubes. Now I am dating myself.

#### Audioguru again

Joined Oct 21, 2019
3,542
I think some old teachers are still teaching about the 53 years old 741 opamp because they never learned about newer and better opamps.
Learning and teaching are different now because of the internet. Now the teachers send the students here.

#### MrAl

Joined Jun 17, 2014
8,479