how to separate AC and DC current signal

[bhl777]

Hi all, I have a stage to generate a AC+DC current signal, and just want to feed the generate AC signal to the next stage, would anybody advise how can I do this? A simple schematic is shown here:

There are three questions,
(1) The first stage generate a AC+DC current signal in transistor Q0, if I just need its AC component, can I simply use a dc current to bias this transistor, and use another branch as the AC signal path?
(2) If this ac signal will be the input of the second stage, what is the interface I need between these two stages? Like the orange box in this picture, if I can export the AC current signal from the first stage, how can I do to feed it to the second stage, without impacting any DC operating points?
(3) If the green box (the second stage) is a RMS-DC conversion, ideally it will generate a DC current at Q1, including both the RMS value of input AC sine signal, and the bias current of Q1. If I can force a DC current as DC1 to bias Q1, how can I export DC2 current? How can I connect with this DC2 wire for further processing?

Thank you!

 

[srizbf]

to hve minimised dc offset when feeding AC+DC from Q0 to second stage , their collector voltages(DC) should be kept at the same value.

 

[bhl777]

to hve minimised dc offset when feeding AC+DC from Q0 to second stage , their collector voltages(DC) should be kept at the same value.

If I can bias both collect at the same value, then can I simply put a resistor between these two nodes, to let AC pass?

 

[JSCV]

If you only want to pass AC, you can use a Capacitor between your 2 stages.
The capacitor will remove all of your DC offset.

 

[srizbf]

If I can bias both collect at the same value, then can I simply put a resistor between these two nodes, to let AC pass?

yes. a resistor can be connected.

 

[bhl777]

yes. a resistor can be connected.

Thank you srizbf, are you familiar with RMS-DC circuits too? If so, would you tell me how can I seperate the DC bias current of the output transistor, and the generated DC value which equals to RMS value of input AC signal?

 

[srizbf]

can you elaborate more ?

... how can I seperate the DC bias current of the output transistor, and the generated DC value which equals to RMS value of input AC signal?

 

[bhl777]

can you elaborate more ?

Hi srizbf, I have figured it out. Thank you all the same!

- - - Updated - - -

If you only want to pass AC, you can use a Capacitor between your 2 stages.
The capacitor will remove all of your DC offset.

Hi JSCV, I have three questions for putting a cap:
1. will it change the impedance matching between two stages? How can we remove the impact of this cap to the DC operating point?
2. How can I design the value of the cap? For example, I want to pass the sine wave between 100kHz to 2MHz.
3. will this cap introduce any phase shift to the input sine wave?
Thank you!