Amplifiers
Cascade Multistage Connection
AC Coupled Multistage Amplifiers
The output of one amplifier is the input to the next amplifier.
The overall gain:
Av Av1 Av 2 Av k Av n
where Z Lk Z i k 1 or
Z s k Z ok 1
Note the DC bias circuits are isolated from each other by the coupling capacitors.
The DC calculations are independent of the cascading.
The AC calculations for gain and impedance are interdependent.
FET Cascade Amplifier
Voltage Gain:
Av Av 2 Av1 g m 2 R D2 g m1 R D1 || RG2
Input Impedance:
Z i RG1
Output Impedance:
Z o R D2
BJT Cascade Amplifier
Input Impedance:
h fe RC
Av Av 2 Av1 2 2
hie 2
Z i R1 || R2 || hie1
Output Impedance:
Z o RC 2 ||
Voltage Gain:
1
hoe 2
h fe1 RC1 || R3 || R4 || hie 2
hie1
Combination of FET and BJT Cascade
A FET-BJT cascade is calculated in a similar fashion as a FET-FET or
a BJT-BJT cascade.
This combination provides a high gain from the BJT with the high input
impedance from the FET.
Ex1:
a) Draw AC and DC load lines for both transistors.
b) Calculate the overall voltage gain AV = vo / vs.
c) Find vs (max) which produces maximum undistorted output voltage.
DC Coupled Multistage Amplifiers
The output of one amplifier is the input to the next amplifier.
The overall gain:
Av Av1 Av 2 Av k Av n where
Z Lk Z ik 1
or
Z s k Z ok 1
Note the DC bias circuits are not isolated from each other
The DC calculations are not independent the of cascading.
Used either to amplify very low frequency signals or to amplify DC signals
Ex2:
a) Draw AC and DC load lines for both transistors.
b) Calculate the overall voltage gain AV = vo / vs.
c) Find vs (max) which produces maximum undistorted output voltage.
Ex3:
a) Draw AC and DC load lines for both transistors.
b) Calculate the overall voltage gain AV = vo / vs.
c) Find vs (max) which produces maximum undistorted output voltage.
Ex4:
b) Calculate the overall voltage gain AV = vo / vs.
c) Find Ro.
Cascode Connection
This is a CE CB combination.
This arrangement provides high input impedance but a low voltage gain.
The low voltage gain reduces the Miller Input Capacitance therefore this combination
works well in high frequency applications.
A more elaborate version of cascode amplifier
Ex5:
VBE = 0.7V
Consider the cascode amplifier above
a) Find the voltage gain Av = vo/vi
b) Find the input resistance Ri
c) Find the output resistance Ro
Darlington Connection
This combination provides large current gain, typically a few thousand.
It has a voltage gain of near 1, a low output impedance and a high input impedance.
Feedback Pair
This is a two-transistor circuit that operates like a Darlington pair.
It has similar characteristics: high current gain, voltage gain of near 1, low output
impedance and high input impedance.
Note: it is not the Darlington configuration:
Darlington: 2 npn BJTs
Feedback Pair: pnp driving an npn BJT