# Source Follower with Current-Source Load

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1. Oct 22, 2014

### arich13

1. The problem statement, all variables and given/known data
I am trying to analyze the following circuit:

"Note that if the two 2N5485s approximately match in their characteristics, not only is the voltage gain unity, but the DC offset is small: the constant current due to Q2 creates a constant voltage drop across R1. Furthermore, since the gate of Q2 is at the same voltage as the bottom of R2, this should also be true for Q1 and R1. This the output voltage must follow the source voltage of Q1." I am trying to understand the overall operation of the circuit, with a focus being on how to figure out that the last sentence is true.

2. Relevant equations

3. The attempt at a solution
I know that the current source provided by the addition of a second FET creates a constant current and has a very high Zout relative to Zin. This causes the constant voltage drop across R1, as mentioned above. I am really struggling to understand the last statement of the paragraph and how that relates to the impedance.

Thanks in advance for any hints you can provide.

Last edited by a moderator: Oct 22, 2014
2. Oct 23, 2014

### Staff: Mentor

Hi Arich13. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

There's a constant current through R1, Q2, and R2. So the P.D. across R1 is constant and fixed, meaning that if the top of R1 undergoes a change in voltage, then the bottom of R1 will (and must) change by the same amount. If this were to not occur, then the current through R1 would not be a constant current.

Last edited by a moderator: May 7, 2017
3. Oct 23, 2014

### arich13

Thank you very much for your help!

4. Oct 23, 2014

### rude man

Since the curremt thru Q1 must stay constant by virtue of the constat-current sink Q2-R2, Vgs1 must also stay constant. Thus, any variation in Vg1 is followed 1:1 by Vs1.