# Implementing feedback control with transistors

#### Matjelo Naleli

Joined Oct 7, 2015
42
I am trying to implement feedback control using transistors (instead of op amps). Part of the reason for this is that the voltages that I want to operate with are around 700V peak-to-peak and I couldn't find an op-amp that can work in that regime.

As can be seen in my circuit I use two amplifiers, one as a summer (for feedback) while the other one is part of the controller gain.
I haven't included the coupling capacitors at the input and output of each amplifier mainly because I thought they would add additional poles and complicate my trnasfer function. But I guess one can choose the size of coupling capacitor such that the dynamics they add are recessive to the desired ones.

My Question:
I would appreciate comments and criticisms of this design and how it can be improved but most importantly if the transfer functions written there are accurate reflections of the circuit itself.

Thanks

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#### crutschow

Joined Mar 14, 2008
24,725
What is the highest frequency of operation?

#### Matjelo Naleli

Joined Oct 7, 2015
42
I am aiming to operate at resonance frequency which is 15MHz

#### crutschow

Joined Mar 14, 2008
24,725
You want 700V at 15MHz??

#### Matjelo Naleli

Joined Oct 7, 2015
42
Yes that's right, 700Vpp @ 15MHz

#### crutschow

Joined Mar 14, 2008
24,725
Good luck with that.

#### RichardO

Joined May 4, 2013
2,271
You want 700V at 15MHz??
I have to ask another question... At how much power?

#### Matjelo Naleli

Joined Oct 7, 2015
42
The current can be as small as possible, there is really no strict demand on current and power. The important thing is the voltage and the operating frequency.. Infact at this moment we can careless about what voltage or power is required but focus on whether the transfer functions (voltage to voltage) shown are accurate representations of the circuit.

When I made this circuit I mainly thought of using the transistor as an voltage amplifier and a voltage summer inorder to implement positive feedback. I did not, at any point, think about the small signal analysis when I build this circuit so I am hoping to get criticisms on how considering the small signal analysis is going to mess up the transfer functions on the image.

Ideally what I wanted to build is a second order transfer function (Low pass or Band pass) and then feed back (positive feedback) the output voltage back at the input.

I would also appreciate comments on which small signal model would be appropriate at 15MHz. I have very limited background on small signal analysis but I hope to learn.

#### kubeek

Joined Sep 20, 2005
5,718
I have no idea why you need 15MHz, at 700V the circuit will be a radio transmitter, and you probably should obtain a license for that radio band. Or you could use the 13.56MHz ISM band which allows for much greater transmit power.