Electronic Devices and Circuits 3
Lecture 5
The Darlington Pair
The Sziklai Pair
Multistage Voltage Gain
AC analysis
Voltage gain
Current gain
Power gain
The Darlington Pair
One way to boost input resistance is to use a Darlington pair, as shown in Figure . The
collectors of two transistors are connected, and the emitter of the first drives the base of
the second. This configuration achieves βac multiplication. The emitter current of the
first transistor is:
This emitter current becomes the base current for the second transistor, producing a
second emitter current of
Therefore, the effective current gain of the Darlington pair is
Neglecting by assuming that it is much smaller than
RE, the input resistance is
The emitter-follower is often used as an interface between
a circuit with a high output resistance and a low-resistance load.
The emitter-follower is called a buffer
The Darlington Pair
The Sziklai Pair
The Sziklai pair, shown in Figure, is similar to the Darlington pair except
that it consists of two types of transistors, an npn and a pnp. This
configuration is sometimes known as a complementary Darlington or a
compound transistor. The current gain is about the same as in the Darlington
pair, as illustrated. The difference is that the Q2 base current is the Q1
collector current instead of emitter current, as in the Darlington arrangement.
An advantage of the Sziklai pair, compared to the
Darlington, is that it takes less voltage to turn it on
because only one barrier potential has to be overcome. A
Sziklai pair is sometimes used in conjunction with a
Darlington pair as the output stage of power amplifiers.
In this case, the output power transistors are both the
same type (two npn or two pnp transistors). This makes it
easier to obtain exact matches of the output transistors,
resulting in improved thermal stability and better sound
quality in audio applications.
Multistage Voltage Gain
The overall voltage gain , of cascaded amplifiers, as shown in Figure, is
the product of the individual voltage gains.
2. The dc source is replaced by ground.
This is why a dc source is called an ac ground.
Multistage Voltage Gain
Capacitively-Coupled Multistage Amplifier
For purposes of illustration, we will use the two-stage capacitively coupled
amplifier in Figure. Notice that both stages are identical common-emitter
amplifiers with the output of the first stage capacitively coupled to the
input of the second stage. Capacitive coupling prevents the dc bias of one
stage from affecting that of the other but allows the ac signal to pass
without attenuation because at the frequency of operation.
Notice, also, that the transistors are labeled Q1 and Q2.
Capacitively-Coupled Multistage Amplifier
Loading Effects In determining the voltage gain of the first
stage, you must consider the loading effect of the second stage.
Because the coupling capacitor C3 effectively appears as a short
at the signal frequency, the total input resistance of the second
stage presents an ac load to the first stage.
The voltage gain of the first stage is reduced by the loading of
the second stage because the effective ac collector resistance of
the first stage is less than the actual value of its collector resistor,
R3. Remember that .
Capacitively-Coupled Multistage Amplifier
Capacitively-Coupled Multistage Amplifier
Direct-Coupled Multistage Amplifiers
A basic two-stage, direct-coupled amplifier is shown in Figure. Notice that
there are no coupling or bypass capacitors in this circuit. The dc collector
voltage of the first stage provides the base-bias voltage for the second stage.
Because of the direct coupling, this type of amplifier has a better low-
frequency response than the capacitively coupled type in which the reactance
of coupling and bypass capacitors at very low frequencies may become
excessive.
Direct-coupled amplifiers can be used to amplify low frequencies all the way
down to dc (0 Hz) without loss of voltage gain because there are no capacitive
reactances in the circuit.
