Source Follower with Current-Source Load

[arich13]

Homework Statement

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.

Homework Equations

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.

 

[NascentOxygen]

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

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.

 

[arich13]

Thank you very much for your help!

 

[rude man]

Since the curremt thru Q1 must stay constant by virtue of the constat-current sink Q2-R2, V_gs1 must also stay constant. Thus, any variation in V_g1 is followed 1:1 by V_s1.

 

[rezeew]

The statement at the end of the paragraph is referring to the fact that the output voltage of the circuit will be equal to the source voltage of Q1. This is because Q1 acts as a buffer, or follower, for the source voltage. In other words, the output voltage will follow the source voltage, hence the name "source follower".

To understand this concept, it may be helpful to think about the impedance of the circuit. The gate of Q1 is connected directly to the source voltage, so there is no voltage drop across the gate-source junction. This means that the voltage at the gate will be equal to the source voltage. Since the gate is connected to the drain of Q1, this also means that the output voltage will be equal to the source voltage.

In terms of impedance, the gate-source junction of Q1 has a very high impedance, so it will not draw any current from the source. This allows the source voltage to be passed through to the output without any attenuation or distortion.

I hope this helps to clarify the last statement and the overall operation of the circuit.