What is the purpose of Q2?

[honorasm]

It is a RF amp taken from the OrCAD simulation examples. What is the purpose of Q2, D1 and C1? I think Q2 is biasing Q1, but can not find out how it works. I don't know why D1 and C1 is needed, either.

 

[poor mystic]

Hi and welcome
Q1 appears to me to be part of a negative feedback loop, which helps keep the output of the amplifier faithful to the input - except that the output is amplified. D1 keeps Q1 in a usefully semiconductive state - that is it's a part of Q1's bias network. I'm thinking that C1 helps provide a low-pass filtering effect in the output.
It's been a while since I looked hard at a transistor circuit so I probably got some of this wrong.
Guys?

 

[Delta Prime]

Welcome to electronics-lab. Here you go, kiddo!

Noise figure in PSPICE

There is an example in OrCad PSPICE which calculates noise figure for an RF amplifier. I know the following formula for noise figure: $$\text{NF}_\text{dB}=10\log\frac{P_{no}}{G_aP_{ni}}$$ But i...

electronics.stackexchange.com

 

[poor mystic]

but what are Q2; C2 and D1 for, @Deltaprime? (They still look like active filter feedback to me)

 

[Delta Prime]

but what are Q2; C2 and D1 for, @Deltaprime? (They still look like active filter feedback to me)

Do not quote me boy… because I have not said anything yet!
This is a very crude RF LNA
Adding D1 in circuit is used for compensating the -2.5mv/C of the base emitter junction of the PNP transistor Q2 reducing thermal instability, noise and maintain the DC bias of the amplifier at a giventemperature range. Resistors (R1, R2, R3, R6, R7, R8, R9) and diode (D1) are used to establish the DC operating points for the transistors.
Capacitors (C1, C3, C4, C6, C7) are used for coupling AC signals and blocking DC signals along with bypassing unwanted frequencies and or noise.
I hope that answers your question “desert shaman!”
https://resources.pcb.cadence.com/b...ng-sensitivity-analysis-in-pspice-part-1-of-4

 

[poor mystic]

Do not quote me boy… because I have not said anything yet!
This is a very crude RF LNA
Adding D1 in circuit is used for compensating the -2.5mv/C of the base emitter junction of the PNP transistor Q2 reducing thermal instability, noise and maintain the DC bias of the amplifier at a giventemperature range. Resistors (R1, R2, R3, R6, R7, R8, R9) and diode (D1) are used to establish the DC operating points for the transistors.
Capacitors (C1, C3, C4, C6, C7) are used for coupling AC signals and blocking DC signals along with bypassing unwanted frequencies and or noise.
I hope that answers your question “desert shaman!”
https://resources.pcb.cadence.com/blog/know-how-your-circuit-works-using-sensitivity-analysis-in-pspice-part-1-

I thought that the OP wanted to know about the purpose of Q2 and the stage that it powers.
Does it seem to others that it is part of the negative feedback network?
(For the OP's sake, if he needs to find this out, a negative feedback network is used to ensure a flat response over a range of inputs.)

 

[Delta Prime]

I thought that the OP wanted to know about the purpose of Q2 and the stage that it powers

You’re too kind… I really should read the title of the post.
Thank you @poor mystic for keeping me “on topic.”to the Thread Starter… Using an incomplete posted schematic as reference, from a circuit simulation software,ORCAD, Candace or PSPICE. Tells me you’re playing with component values and your well established in transistors 101!
If not, this is a good read…

https://www.allaboutcircuits.com/technical-articles/introduction-to-rf-class-a-power-amplifiers/

 

[poor mystic]

I keep changing my mind about Q2. The circuit's operation only gradually emerges to me through the murkiest of mists; have I got it right yet?
There's already negative feedback around Q1, of course. That's provided by R4 and C7.
I can't see Q2 as anything more than active bias from my understanding right now. That is unless this is some magic circuit that only works in simulation, and is super-dicky about circuit values, and only works at 1 frequency.
Anyone?

and why should Q1 need active bias anyway?

 

[poor mystic]

OK, that's how you want it.... but might not a "fellow contributor", such as the OP, such as myself (and even such as yourself, potentially) ask questions? I'm sure the questions are obvious enough.
Firstly the OP wants to know about Q2, and secondly I've proposed an answer, which is also in question.
I'm primarily trying to communicate with the OP, and I don't see why they should have to put up with people squabbling over very little. The OP hasn't had their answer very clearly yet.

Please, has anyone else anything to say?