# Need help with this cascode circuit

Discussion in 'Homework Help' started by milanyour12, Sep 29, 2014.

1. ### milanyour12 Thread Starter New Member

Apr 3, 2013
12
0
I am trying to solve the attached question related to cascode circuit. Where T5 has Vbe=0.7V

I have tried to answer the questions as follows but I am not sure if its correct.

- Explain how the circuit works? The circuit has a DC operating point which gives the AC parameters of the transistors such as the transconductance. A AC current will flow through the transistor and the output voltage will be given by the voltage dropped at R4. The input impedance of the circuit is high but it's not depending on the bias current due to the gate current is really small, thus the rd can be regard as infinite.

- Explain the FET T3 is biased at the red point in figure 3 characteristics. how do I explain this, does this has something to do with calculating the Q point in the load line?

- Calculate the value of R3 using the data from figure 3.

Vgs = Id * R3 there fore,

R3 = Vgs/Id = 0.4V/10mA = 40 ohm

- **Determine in a graphical way from the characteristics from figure 3 the transconductance of T3?**

gm = delta Id/delta Vgs (keeping Vds constant)

gm = (13.8-6.3)/(0.2-0.6)

gm = 7.5mA/0.4 = 18.75 mA/V

- Calculate the voltage gain of the circuit?

Vout = R4 * id

We know, id = gm * Vgs = gm*(vin - id*R3) = (gm *vin)/(1+R3*gm)

Replacing the value of id we get,

Vout = (gm*R4*vin)/(1+R3*gm)

Vout/vin = (gm * R4) /(1+R3*gm)

Av = (18.75mA/V * 3.9K ) / (1 + 40*18.75mA/V)

= 41.78

- Judge the power supply rejection of circuit 3? This is a small signal analysis so i think,

PSRR = 20log ((delta Supply / delta Vout)*AV)

but i don't have the supply voltage and Vout so how can I calculate PSRR?

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,167
1,188
Hi, to be honest I don't like your explanation. But do you explain how the circuit work ? Or how self biasing work?
As for the PSRR, are you sure you need to know VDD and Vout ?? I see a delta in the equation, not the constant values.