Analogue Electronics (ECE)
(15B11EC411)
Lecture 06
By
Dr. Archana Pandey
Source- A.S .Sedra & K.C.Smith, Microelectronic CIRCUITS Theory and Application, 6th Edition,
Oxford University Press, 2011 (Text Book)
Outline
Module 1- BJT Amplifier
BJT circuits at DC - -Lecture 01
Small-signal models (pi and T-model)-Lecture 02,03
Analysis of Basic BJT Amplifier configurations
➢ Common Emitter (CE) -Lecture 03
➢ Common Emitter (CE) with RE –Lecture 04
➢ Common Base (CB)- Lecture 04
➢ Common Collector (CC)- Lecture 05
➢ Analysis of Basic BJT Amplifier configurations with coupling
capacitors- Lecture 06
Cascode configuration (CE-CB)- Lecture 06
High and Low Frequency response of CE
Characteristics of BJT amplifiers
The Common-Emitter (CE) Amplifier
Figure (a) shows a CE amplifier implemented using coupling capacitors.
To establish a signal ground (or an ac ground, as it is sometimes called) at the
emitter, a large capacitor CE, usually in the range of microfarads or tens of
microfarads is connected between emitter and ground.
This capacitor is required to provide a very low impedance to ground (ideally,
zero impedance; i.e., in effect, a short circuit) at all signal frequencies of
interest.
In this way, the emitter signal current passes through CE to ground and thus
bypasses the output resistance of the current source I (and any other circuit
component that might be connected to the emitter);
Hence CE is called a bypass capacitor. Obviously, the lower the signal frequency,
the less effective the bypass capacitor becomes.
The Common-Emitter (CE) Amplifier…
Figure (a) A common-emitter amplifier using coupling capacitors
The Common-Emitter (CE) Amplifier…
Figure (b) Equivalent circuit obtained by replacing the transistor with its hybrid-π model
The Common-Emitter (CE) Amplifier…
For our purposes here we shall assume that CE is acting as a perfect short
circuit and thus is establishing a zero signal voltage at the emitter.
In order not to disturb the dc bias currents and voltages, the signal to be
amplified, shown as a voltage source vsig with an internal resistance Rsig, is
connected to the base through a large capacitor CC1.
Capacitor CC1, known as a coupling capacitor, is required to act as a perfect
short circuit at all signal frequencies of interest while blocking dc.
The voltage signal resulting at the collector, vc, is coupled to the load resistance
RL via another coupling capacitor CC2.
We shall assume that CC2 also acts as a perfect short circuit at all signal
frequencies of interest; thus the output voltage vo = vc.
Note that RL can be an actual load resistor to which the amplifier is required to
provide its output voltage signal, or it can be the input resistance of a
subsequent amplifier
The Common-Emitter (CE) Amplifier…
To determine the characteristic parameters of the CE amplifier, that is, its input
resistance, voltage gain, and output resistance, we replace the BJT with its
hybrid π , small-signal model.
The resulting small-signal equivalent circuit of the CE amplifier is shown in Fig.
(b).
The equivalent circuit of Fig. (b) can be used to determine the amplifier
characteristic parameters in exactly the same way we used for the “stripped-
down” version of the CE amplifier in last lectures
Some of the analysis is done directly on the circuit itself in Fig. (a).
Observe that the only difference between the circuit in Fig. (b) and the
simplified version of previous lectures is the bias resistance that appears across
the amplifier input and thus changes Rin to (Rin=RB||rπ)
The Common-Emitter (CE) Amplifier…
Apply the Thévenin theorem to the network composed of
vsig, Rsig, and RB thus reducing it to a generator
vsig’=(RB/(RB+Rsig))vsig and a resistance Rsig’’= Rsig||RB
Now the formulas previously obtained for the Common
emitter amplifier can be modified as-
➢ Replace the expression for Rin
➢ Multiply the expression for Gv by the factor RB/(RB+Rsig)
➢ Replace Rsig in that expression by Rsig||RB
The Common-Emitter Amplifier with an
Emitter Resistance
Figure (a) A common-emitter amplifier with an emitter resistance Re.
The Common-Emitter Amplifier with an
Emitter Resistance
Figure (a) A common-emitter amplifier with an emitter resistance Re, Equivalent
circuit obtained by replacing the transistor with its T model.
The Common-Base (CB) Amplifier
Figure (a) shows a CB amplifier with coupling capacitors.
Observe that since both the dc and ac voltages at the base are zero, we have
connected the base directly to ground, thus eliminating resistor RB altogether.
Coupling capacitors CC1 and CC2 perform similar functions to those in the CE
circuit.
The small-signal, equivalent-circuit model of the amplifier is shown in Fig. (b).
This circuit is identical to that for the simplified version we obtaine din
previous lectures
Thus the analysis of previous lecture, and indeed the results summarized for the
common base amplifier simplified version apply here directly
The Common-Base (CB) Amplifier
Figure (a) A common-base amplifier showing the coupling capacitors.
The Common-Base (CB) Amplifier
(b) Equivalent circuit obtained by replacing the transistor with its T model
The Emitter Follower
An emitter-follower circuit showing the coupling capacitors
is shown
Observe that since the collector is to be at signal ground, we
have eliminated the collector resistance RC.
The input signal is capacitively coupled to the base, and the
output signal is capacitively coupled from the emitter to a
load resistance RL.
The Emitter Follower…
Figure (a) An emitter-follower circuit showing the coupling capacitors
The Cascode Amplifier
Cascoding refers to the use of a transistor connected in the
common-gate (or the common-base) configuration to provide
current buffering for the output of a common-source (or a
common-emitter) amplifying transistor.
Figure (a) shows the BJT cascode amplifier with an ideal current-
source load. Voltage is a dc bias voltage for the CB cascode
transistor
Our objective then is to determine the parameters and of the
equivalent circuit of Fig. (b), which we shall use to represent the
output of the cascode amplifier formed by Q1 and Q2
Here we show the cascode amplifier prepared for small-signal
analysis with the output short-circuited to ground. The
transconductance can be determined as
The Cascode Amplifier…
Figure (a) A BJT cascode amplifier with an ideal current-source load; (b) small-signal
equivalent circuit representation of the output of the cascode amplifier; (c) the cascode
amplifier with the output short circuited to ground,
The Cascode Amplifier…
(d) equivalent circuit representation of (c).
The Cascode Amplifier…
To obtain Ro, we set
vi=0 which results in
Q1 being reduced to its
output resistance ro1,
which appears in the
emitter lead of Q2
Apply a test voltage to
determine Ro as vx/ix
The Cascode Amplifier…
The Cascode Amplifier…
Determining the output resistant Ro of the BJT cascode amplifier
The Cascode Amplifier…
• Before the analysis, it is very useful to observe first that the current flowing into the
emitter node must be equal to ix,
• Second, note that ro1 and rπ2 appear in parallel. Thus the voltage at the emitter
node -v π2, can be found as
Output Resistance
Thanks
