Due Wednesday Feb 20 at the start of class

folded cascode
Figure 1. Folded cascode amplifier
folded cascode ac
Figure 2. Folded cascode amplifier — AC equivalent circuit

1. Draw the small-signal equivalent circuit

(this was done in-class Monday)

2. Linear circuit analysis

Consider the three capacitors Cin, Cb, and Cout as “BFC’s” — i.e. large enough to be effectively short circuits at all signal frequencies. It will be acceptable to ignore \(r_o\) (consider an open circuit) for Q2, it has little numerical influence in this circuit. Use this small-signal equivalent circuit to find symbolic solutions for:

  • \(v_2\) as a function of \(v_1\)

  • \(v_{out}\) as a function of \(v_2\)

  • \(v_{out} / v_{in}\)

  • \(v_{out} / v_{s}\)

Use the resistor values determined from Homework 07 to compute the transistor small-signal parameters \(r_\pi\), \(r_o\), and \(g_m\) for each transistor with the help of Tour Book Table 4. Small-signal parameters and Tour Book Table 2. Transistor parameter definitions . Then compute the overall amplifier’s small-signal input and output resistances and open-circuit voltage gain. Remember that the definition of Av0 is the voltage gain from \(v_{in}\) (not Vs!) to \(v_{out}\) with the load disconnected.

For help determining where, exactly, the amplifier input and outputs are, consider capacitors Cin and Cout as connecting the signal source and load to the amplifier’s proper input and output, respectively.

Table 1. Folded cascode amplfier major parameters
Device Value (units)

Rin

Rout

Av0

You can cross-check these values by computing them via a different route by using the “table technique” with Tour Book Table 6. Bipolar transistor amplifier types. They should also match your measurement results for Lab 3-B.