Amplifier transistor circuit (positive feedback)

[yalect]

Hello,

I would like to ask you if a capacitor connected between the collector and  the Emitter of transistor can's work like positive feedback as shown on my attached circuit or that can’s decrease the output signal? if it doeshow it’s calculated?

Thank you

[johnsold]

Considering that the output capacitance of Q1 is larger than C2, I would not expect much effect. 

When I simulate that circuit (using 2N2222A transistor) I see no effect from C2. Increasing C2 to 40 pF reduces the high frequency cutoff from 3 MHz to 2.5 MHz.  In the image below the white trace is the input and the colored trace is the output.

Lynn

[yalect]

Hello,

Thank you John for your reply and care  

I think that some electronic simulator could not give a true results

As shown in your explication , if we take a part of the output signal from the collector to the emitter of the transistor will not increase the value of the output, otherwise the capacitor C2 works like filter pass low

regards

[yalect]

Hello, Mr john

did you read my reply ?

thank you

[johnsold]

I am not sure i understand your question.

Simulations are never better than the models used.  Your schematic did not specify a type for D1. I used the same transistor model as for Q1 with the collector tied to the base. I did not have a model for the BC547A so I used a 2N2222A.

The capacitor C2 is parallel to the current source in a small signal model of the transistor. So it will shunt some of the signal from the output.

A circuit simulator is not a substitute for understanding how a circuit works. It is a tool for calculating how well it works.

Lynn

[yalect]

Hello,

Thank you for your reply

Yes, I am agree with you a circuit simulator

Basing on your simulation you did, the capacitor connected between the emitter and the collector of the tansistor doesn’t work like a positive feedback ?

There are some circuit have the same base of working means feedback from the collector to the emitter of transistor as the following circuit which build to work like an RF reciever

So, C2 is getting  a part of the output signal into the emitter, here I want to ask you as a second part of this question (if it’s possible) for my second circuit , how to calulate C2 and increasing the L1 increases the value of the feedback ?

regards    

[yalect]

Hello,

Might you don’t understand my question?

Briefly, in my first circuit basing on your simulation you did, C2 could not work like a positive feedback

I gave second circuit as an example for circuit has almost the same base of working may we will discuss about later   

regards 

[yalect]

Hello, Mr john

Did you read my replies?

Thank you