Facts Flashcards
What are the 5 classifications of linear amplifiers
input
output
frequency response
biasing conditions
transistor configuration
low
medium
high
audio -
Intermediate -
radio -
What happens to a CB transistor when a small signal is applied?
When positive half-cycle of the signal is applied, then
1. .forward bias is decreased because VBE is already negative with respect to the ground as
per biasing rule of Art. 60.3.
2. consequently, IB is decreased.
3. IE and hence I
C are decreased (because they are both nearly β times the base current).
4. the drop IC RC is decreased.
5. hence, V CB is increased as seen by the equation given above.
Give two uses of CB amplifier
One of the important uses of a CB amplifier is in matching a low-impedance circuit to a high impedance circuit.
It also has high stability of collector current with temperature changes.
Give 6 characteristics of CE amplifier
- it has moderately low input resistance (1 K to 2 K),
- its output resistance is moderately large (50 K or so),
- its current gain (β) is high (50–300)
Explain the circuit operation of a CE amplifier
When positive half-cycle of the signal is applied (Fig. 60.4)
1. VBE is increased because it is already positive w.r.t. the ground as per biasing rule
of Art 6.3.
2. it leads to increase in forward bias of base emitter junction
3. IB is increased somewhat
4. IC is increased by α times the increased
in IB.
5. drop IC RC is increased considerably and
consequently.
6. V CE is decreased as seen from the equation given above
Uses of CE amplifier
Most of the transistor amplifiers are of CE type because of large gains in voltage, current and power. Moreover, their input and output impedance characteristics are suitable for many applications.
Characteristics of a CB Amplifier
- very low input resistance (30 – 150 Ω)
- very high output resistance (up to 500 K)
- a current gain α < 1
- large voltage gain of about 1500
- power gain of up to 30 dB
- no phase reversal between input and output voltages.
How does Common Collector (CC) Amplifier
work?
- forward bias is increased since VBE is positive w.r.t. collector i.e. ground,
- base current is increased,
- emitter current is increased,
- drop across RE is increased,
- hence, output voltage (i.e. drop across RE is
increased.
Characteristics of a CC Amplifier
- high input impedance (20-500 K)
- low output impedance (50-1000 Ω),
- high current gain of (1 + β) i.e. 50 – 500
- voltage gain of less than 1
- power gain of 10 to 20 dB
- no phase reversal of the input signal.
Uses of CC amplifiers
- for impedance matching i.e. for connecting a circuit having high output impedance to one
having low input impedance; - for circuit isolation;
- as a two-way amplifier since it can pass a signal in either direction;
- for switching circuits
Explain the Amplifier Classifications Based on Biasing Conditions
(a) Class-A Amplifier
In this case, the transistor is so biased that output
current flows for the fullcycle of the input signal
(360°) as shown in Fig. 60.11
(a). In other words, the transistor remains FR-biased
throughout the input cycle.
Hence, its conduction angle
is 360°.
(b) Class-B Amplifier
In this case, the transistor bias and the amplitude of
input signal are such that output current flows for only
half-cycle (180°) of the input signal. It means that transistor stays FR-biased for half the input cycle. The transistor conduction
angle equals 180°.
(c) Class-C Amplifier
In this case, transistor bias and signal amplitude are such that output current flows for appreciably
less than half-cycle of the input signal i.e. upto 120 ° or 150° angle of conduction as shown
in 60.11 (c). In other words, transistor remains FR-biased for less than half the cycle.
(d) Class-AB Amplifier
The characteristics of such an amplifier lie in-between those of class-A and class-B. Here,
biasing conditions are such that output current flows for appreciably more than half but less than the
entire cycle i.e. current flows for more than 180° but less than 360°
Give atleast 4 characteristics of Class-A amplifiers
- Since the transistor operates over the linear portion of the load line, the output waveform is
exactly similar to the input waveform. Hence, class-A amplifiers are characterized by a
high fidelity of the output. They are used where linearity or freedom from distortion is the
prime requisite. - Since its operation is restricted only over a small central region of the load line, this amplifier is meant only for amplifying input signals of small amplitude. Large signals, will shift
the Q-point into non-linear regions near saturation or cut-off and produce distortion (Art.
60.30). - Due to the limitation of the input signal amplitude, ac power output per active device (i.e.
transistor) is small. - The overall efficiency of the amplifier circuit is
ac power delivered the load averageac power output = = total power delivered bydcsupply average dc power input. - The collector efficiency of a transistor is defined as
averageac power output = averagedc power input to transistor
The maximum possible collector efficiency of a class – A amplifier with resistive load is
50% - In case an output transformer is used, the maximum possible overall effici
Power Distribution
Fig. 60.15 (a) shows a CE connected transistor which forms the active element of a singles tage class-A amplifier. Fig. 60.15 (b)
shows its output characteristic with a
centered Q-point. When ac input signal is
applied, Q-point shifts up and down from
its central position. The output current will
also increase or decrease from its quiescent (or no-signal) value ICQ. Similarly,
collector-emitter voltage V CEwill increase
or decrease from its quiescent value V CEQ.
So long as signal variations are confined
to linear region of the load line, average
value of collector current is ICQ because
positive and negative input signal swings
will produce equal changes in ICQ.
Hence, total average dc power
drawn by the circuit from collector battery V CC is
Pin(dc) = V CC. ICQ
Now, this power goes to supply the
following :
(i) heat dissipated by the load resistor RC connected to the collector
PRC(dc) = RC
(ii) the balance Ptr(dc) is given to the
transistor.
It is further subdivided into :
(a) ac power developed across the
load resistor which constitutes
the ac power output
2 2
2
2
m o(ac) C
C C
V V
P IR
R R = ==
where I is the rms value of the ac output current through the load, V is rms value of ac voltage and
V m is its maximum value.
(b) power dissipated (in the form of heat) by the transistor itself i.e. its collector region. It may
be called Pc(dc).
Since, under zero-signal condition, there is no ac output power, all the power given to the
transistor is wasted as heat. Hence, a transistor dissipates maximum power under zero-signal
condition.
The power flow diagram of the transistor is as follows
Draw the power flow diagram of the transistor
x
