Transimpadence Amplifier with 2N3904

[kampfer]

I made this circuit for project. yet i encountered some problems. i am not sure about gain resistance specs.



I need 5 Vpp at the output and 6kΩ transimpedance gain. i can get a 5v with 0.86mA ac, however gain is 5.8k. how can i enhance gain?

 

[FvM]

The bias point doesn't look right for 5 Vpp output. If you achieve 5V with the present circuit, the output will be rather distorted.

But you shouldn't expect too much from this simple circuit. You can adjust the gain with R6 and the bias point with R4.

 

[LvW]

What is the purpose of R7 ?

 

[kampfer]

normally, with feedback like this, should i get 6k gain directly? i mean gain is all up to feedback. is it right?

 

[kampfer]

What is the purpose of R7 ?

it keeps gain high and prevent the error "node is floating".

 

[FvM]

normally, with feedback like this, should i get 6k gain directly?

No, only if transistor had inifinite gain.

 

[kampfer]

No, only if transistor had inifinite gain.

books and real life doesn't fit then, eh?
how can i get get 5v with 0.83mA?

 

[LvW]

normally, with feedback like this, should i get 6k gain directly? i mean gain is all up to feedback. is it right?

Even in case of infinite transistor current gain this wouldn`t be true because of the finite source resistance at the collector node.

 

[kampfer]

Even in case of infinite transistor current gain this wouldn`t be true because of the finite source resistance at the collector node.

is there a simple way to see direct output value? i make a ac sweep with iac and it gives 4.9V; is it peak-to-peak?
and when i make a trasient with Isin source it gives 2.4V to top but i gonna cut-off at the negative values.

 

[kampfer]

this is ac sweep with 0.833mA ac source



this is trasient with Isin 0.833mA source

 

[FvM]

Review what I said in post #2 about bad bias point and a possible solution.

 

[kampfer]

Review what I said in post #2 about bad bias point and a possible solution.

what is the reason of cut-off in negative values?

 

[LvW]

what is the reason of cut-off in negative values?

According to your first post the dc operating point is app. at +1.5 volts.
That means: The voltage can swing araound this point with a max. amplitude of 1.5 volts approximately (3 volts peak-peak).
Now - look at your transient simulation results and verify that the peak-peak value is app. 3 volts.
Do you now understand FvM`s recommendation?