# how can i make temperature constant amplifier with bjts?

#### songhan

Joined Jun 8, 2021
19
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.
He did not teached anything about stock amplifiers. He just taught bjt and negative feedback with ideal-opamp..

#### MrAl

Joined Jun 17, 2014
8,479
He did not teached anything about stock amplifiers. He just taught bjt and negative feedback with ideal-opamp..
What did he teach you about bjt's so far?

#### songhan

Joined Jun 8, 2021
19
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!
sir i studied how the circuit you gave to me works and about neagtive feedback, and got a question.

picutre1

picture 2

I dont understand why R3 is at base part of picture 2. It seems that without resistance at base still negative feedback works.

thank you!

#### MrAl

Joined Jun 17, 2014
8,479
sir i studied how the circuit you gave to me works and about neagtive feedback, and got a question.
View attachment 240952
picutre1

View attachment 240953
picture 2

I dont understand why R3 is at base part of picture 2. It seems that without resistance at base still negative feedback works.

thank you!
You can get good linearity if you couple Eout to the emitter of Q1 in the first circuit. R3 in that first circuit also provides for negative feedback all by itself except then the bypass cap should have resistance in series with it.

I can tell you that without R3 in the second circuit the circuit drive becomes very stiff. That's because there is an input capacitor and a bypass capacitor making the drive couple directly to the base with little resistance, and that is usually not good except in maybe a voltage follower. It depends though, on the drive stiffness and the levels and the transistor internal specifications.

#### songhan

Joined Jun 8, 2021
19
What did he teach you about bjt's so far?
well pi-model, t-model, how it works as simple amplifer, how to substitute it with current controlled current source. Something normals thing that learn in Electric circuit in sophomore.

#### songhan

Joined Jun 8, 2021
19
You can get good linearity if you couple Eout to the emitter of Q1 in the first circuit. R3 in that first circuit also provides for negative feedback all by itself except then the bypass cap should have resistance in series with it.

I can tell you that without R3 in the second circuit the circuit drive becomes very stiff. That's because there is an input capacitor and a bypass capacitor making the drive couple directly to the base with little resistance, and that is usually not good except in maybe a voltage follower. It depends though, on the drive stiffness and the levels and the transistor internal specifications.
umm i dont still get the meaning of circuit drive very stiff.. maybe i should sleep on that a while..

#### MrAl

Joined Jun 17, 2014
8,479
umm i dont still get the meaning of circuit drive very stiff.. maybe i should sleep on that a while..
The word "stiff" comes up now and then in electronics and math, notably differential equations, but here is simply means stiff as the relative measurement of a response versus some sort of excitation.
For example, a branch from a tree can be very limp or very stiff. If it is limp when you try to bend it it will bend easy and not snap in half. If it is stiff then when you try to bend it it will bend a little but then suddenly snap in half. With reference to the drive, the excitation is the voltage and the response is the current. So the input voltage is the excitation and the response is the input current to the base of the transistor which means the current flows mostly through the base emitter junction. That junction could be considered stiff because when you increase the voltage slightly you get a disproportionate increase in current. Capacitors can also be considered stiff with regard to a voltage change because even a tiny change in voltage can cause a large current through the cap. Since the input source voltage connects to a cap and that cap connects to the base and the emitter connects to another cap to ground and the source voltage is also referenced to ground, for a small change in input voltage we see a relatively large change in input current which is also the base current because the bias resistors and emitter resistor are of a much higher impedance than the caps and base emitter diode for changes in input voltage.
Now consider placing a resistor in SERIES somewhere in that series circuit (input voltage to cap to base emitter to cap to ground). That series circuit becomes "softer" because now a small increase change in voltage has less effect on the increase in input current.
The overall effects of a stiff circuit in an amplifier can be high or at least higher distortion.
In this context 'stiff' and 'soft' are relative terms where the other components are compared to the stiff part. It isnt stiff if the other parts of the circuit are also somewhat stiff, so it is all about relative impedances and in this case it is all about amplitude and impedances when sometimes it is all about the relative time constants.

#### songhan

Joined Jun 8, 2021
19
The word "stiff" comes up now and then in electronics and math, notably differential equations, but here is simply means stiff as the relative measurement of a response versus some sort of excitation.
For example, a branch from a tree can be very limp or very stiff. If it is limp when you try to bend it it will bend easy and not snap in half. If it is stiff then when you try to bend it it will bend a little but then suddenly snap in half. With reference to the drive, the excitation is the voltage and the response is the current. So the input voltage is the excitation and the response is the input current to the base of the transistor which means the current flows mostly through the base emitter junction. That junction could be considered stiff because when you increase the voltage slightly you get a disproportionate increase in current. Capacitors can also be considered stiff with regard to a voltage change because even a tiny change in voltage can cause a large current through the cap. Since the input source voltage connects to a cap and that cap connects to the base and the emitter connects to another cap to ground and the source voltage is also referenced to ground, for a small change in input voltage we see a relatively large change in input current which is also the base current because the bias resistors and emitter resistor are of a much higher impedance than the caps and base emitter diode for changes in input voltage.
Now consider placing a resistor in SERIES somewhere in that series circuit (input voltage to cap to base emitter to cap to ground). That series circuit becomes "softer" because now a small increase change in voltage has less effect on the increase in input current.
The overall effects of a stiff circuit in an amplifier can be high or at least higher distortion.
In this context 'stiff' and 'soft' are relative terms where the other components are compared to the stiff part. It isnt stiff if the other parts of the circuit are also somewhat stiff, so it is all about relative impedances and in this case it is all about amplitude and impedances when sometimes it is all about the relative time constants.
Wow your elaboration is crystal clear.. made me fully understand what that means now. Now i got some ideas what to do. I won't forget your help. One day i wanna be some like you helping others. I really appreciate your help!!

#### MrAl

Joined Jun 17, 2014
8,479
Wow your elaboration is crystal clear.. made me fully understand what that means now. Now i got some ideas what to do. I won't forget your help. One day i wanna be some like you helping others. I really appreciate your help!!
Oh you are welcome and thanks for the kind words.

if you have that kind of interest, and it seems that you do, then if you keep learning you will one day know very much about circuits like this and even learn how to answer the questions you come up with yourself by yourself.