# how can i make temperature constant amplifier with bjts?

#### songhan

Joined Jun 8, 2021
19
Hi i am working on a project and making a amplifier which has constat gain when -50~90 temperature change with bjts.
Is there any way that i can make simple amplifier with constant gain with different temperature?

#### Ian0

Joined Aug 7, 2020
8,425
Negative feedback.

(post some more details such as input and output voltages, gain, output power, frequency response, for a more detailed reply!)

#### Dave Lowther

Joined Sep 8, 2016
187
What does "-50~90 temperature change" mean exactly. I assume it means a change from -50C to + approx 90C, but it could be interpreted in other ways.

#### ronsimpson

Joined Oct 7, 2019
2,756
making a amplifier
Post the schematic of what you have done so far.
If you don't understand "negative feedback" ask questions.

#### songhan

Joined Jun 8, 2021
19
What does "-50~90 temperature change" mean exactly. I assume it means a change from -50C to + approx 90C, but it could be interpreted in other ways.
umm sorry sir.
I mean that intended operation temperature range is up to 90 from -50 celsius. So i want the same gain at the 90 celsius and -50 celcious.

I am wonder if i can give negative feedback in my circuit when temperature changes like the similar way of voltage follower does.

#### songhan

Joined Jun 8, 2021
19
Negative feedback.

(post some more details such as input and output voltages, gain, output power, frequency response, for a more detailed reply!)
Thanks for courtous reply! It is confused how i can give negative feedback against temperature change.
I have no idea of making negative feedback against temperatur change..
If you're willing to give more speicific answer, i will show what i have done with ltspice.

Thank you so much

#### Ian0

Joined Aug 7, 2020
8,425
Thanks for courtous reply! It is confused how i can give negative feedback against temperature change.
I have no idea of making negative feedback against temperatur change..
If you're willing to give more speicific answer, i will show what i have done with ltspice.

Thank you so much
It goes like this:
You make the gain of the amplifier very large (but it still varies with temperature) - call it "A"
Apply negative feedback to set the gain to the value you need. The gain is the ratio of two resistors. The resistors do not vary (significantly) with temperature. Call the ratio of the two resistors "β" (the smaller divided by the larger)
The overall gain of your amplifier with feedback is A/(1+Aβ) which will remain very close to 1/β even if A varies with temperature.

#### songhan

Joined Jun 8, 2021
19
It goes like this:
You make the gain of the amplifier very large (but it still varies with temperature) - call it "A"
Apply negative feedback to set the gain to the value you need. The gain is the ratio of two resistors. The resistors do not vary (significantly) with temperature. Call the ratio of the two resistors "β" (the smaller divided by the larger)
The overall gain of your amplifier with feedback is A/(1+Aβ) which will remain very close to 1/β even if A varies with temperature.
Oh Thank you sir.
I will try. Its still wonder that i can make big gain A with only bjts but anyway it worth a try.
Thank you!

#### BobTPH

Joined Jun 5, 2013
7,564
Even if the feedback resistors vary with temperature, the ratio of them will not if they vary the same way. And the gain depends on the ratio.

Bob

#### BobTPH

Joined Jun 5, 2013
7,564
Oh Thank you sir.
I will try. Its still wonder that i can make big gain A with only bjts but anyway it worth a try.
Thank you!
One word: opamp, or is that two words?
Bob

#### Ian0

Joined Aug 7, 2020
8,425
Even if the feedback resistors vary with temperature, the ratio of them will not if they vary the same way. And the gain depends on the ratio.

Bob
The cheapest 1206 resistor I could find varies at 100 parts per million per degree Celsius. That's 1.4% over the entire temperature range, and only then if one of the pair varies by a different amount to the other.

#### Ian0

Joined Aug 7, 2020
8,425
One word: opamp, or is that two words?
Bob
Of, if you insist on tranistors - BCM857, BCV61, BC817 - open loop gain can exceed 100dB. (I'll draw the circuit once we know the input/output/psu voltages and frequency response required)

#### MrAl

Joined Jun 17, 2014
10,620
Oh Thank you sir.
I will try. Its still wonder that i can make big gain A with only bjts but anyway it worth a try.
Thank you!
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.

#### BobTPH

Joined Jun 5, 2013
7,564
Is the signal AC or DC? If DC, you pretty much need an opamp.

Bob

#### Ian0

Joined Aug 7, 2020
8,425
Is the signal AC or DC? If DC, you pretty much need an opamp.

Bob
because some matched transistors will cost you more than an op-amp.

#### BobTPH

Joined Jun 5, 2013
7,564
He could build an opamp from bjts, but it is unlikely to outperform an IC opamp.

Bob

#### DickCappels

Joined Aug 21, 2008
10,068
Once I had to design a circuit that worked within spec from about -20° to 40°. I designed the circuit then spent the next three days making it work. Transistors (both FETs and Bipolar) experience huge drops in gain at low temperatures.

Depending upon the precision you are looking for, you might find the -50° spec very hard to meet. I suggest checking out the cost and availability of an environment test chamber that can do down to that temperature.

#### songhan

Joined Jun 8, 2021
19
Once I had to design a circuit that worked within spec from about -20° to 40°. I designed the circuit then spent the next three days making it work. Transistors (both FETs and Bipolar) experience huge drops in gain at low temperatures.

Depending upon the precision you are looking for, you might find the -50° spec very hard to meet. I suggest checking out the cost and availability of an environment test chamber that can do down to that temperature.
actually i am making the circuit on simulation(ltspice) so i don't need to consider the cost. Also specification of the bjt is all stated. So i just wanted to know some normal ways(maybe i can say it simple ways because it is just undergraduate level project) to compensate the temperature effect when we make amplifier with bjts.

#### songhan

Joined Jun 8, 2021
19
because some matched transistors will cost you more than an op-amp.
its AC sir, amplifying small siganal which has 10mV amplitude.

#### Ian0

Joined Aug 7, 2020
8,425
its AC sir, amplifying small siganal which has 10mV amplitude.
At what frequency? And how much gain do you need?