Amplifiers: Types, Characteristics and Performance Tests

11/11/2018
NAME

1 Solution A
Amplifiers are the electronic circuits that increases the amplitude of any input signal without
affecting its frequency and phase. Based upon the nature of the output amplifiers can be
divided as,
1) Voltage amplifiers: They helps to increase the amplitude of the input voltage hence
the gain is high. They have a high input impedance and low output impedance. It is
used in medical areas, TV, Air-conditioners etc.
I. Operational Amplifiers: Operational amplifiers are voltage amplifiers which
provide high gain in voltages. They are used for mathematically calculating
voltage values. They have two pins for input, one is the inverting while the
other is the non-inverting. The external connection of resistors and capacitors
can change the amplification form such as inverting, non-inverting, summing,
comparator, voltage follower, differential, integrator and many more. There is
a third pin for output in order to sink and source either voltage or current or
both. An ideal operational amplifier has an infinite loop gain, infinite input
impedance and therefore zero input current, zero offset voltage, infinite output
voltage range, infinite bandwidth with 0 phase shift and zero output
impedance.
A. Inverting Operational Amplifier: In this the non-inverting terminal is
connected to ground and the inverting terminal to the input source via
resistor R1 as shown in figure 1. There is another resistor R2 that
connects the output back to the input forming a feedback resistor. The
values chosen are 1K and 10K for R1 and R2 respectively. In this case
the output will be negative when the input is positive and vice versa.
Suppose the input is 1 volts then the current through R1 will be 1 mA
and the output has to be -10 so that it can supply the same current
maintaining zero voltage at the inverting terminal. The voltage gain
would be R2/R1 = 10K/1K =10. The waveform obtained when a
sinusoidal input of 1 kHz is given to it is shown in figure 2.

Figure 1
Figure 2
B. Non-Inverting Operational Amplifier: This is the reverse of inverting
operational amplifier in which the input is connected to the non-
inverting terminal while the inverting terminal in attached in between
two resistors R1 and R2. The output is always in phase with the non-
inverting terminal keeping the inverting terminal voltage equal to the
non-inverting one. The gain is always greater than 1 (1+R2/R1). The
circuit is shown in figure 3.

Figure 3
C. Summing Amplifier: It is shown in the figure 4 and 5. A summing
operational amplifier basically adds up any number of signals given as
input. In the figure 4, three inputs are taken and in inverting mode. The
three inputs are connected to the inverting terminal via three input
resistors R a, R b and R c. R f is the feedback resistor that connects the
output terminal to the common input point (Circuits Today, 2011). The
non-inverting terminal is connected to ground through a resistor R m.
The load resistor R L is connected to the output pin as shown.
Figure 4

Figure 5
D. Voltage follower: This circuit provides a low output impedance, high
input impedance. The output and the inverting voltages changes when
the input changes. It is shown in figure 6.

Figure 6
2) Current amplifiers: They helps to increase the amplitude of the input current hence the
gain is high. The current gain is constant, it is independent on the conditions of
temperature and humidity, the input impedance is low while the output high. They are
used to increase the bass, used in water and LASER cutting machines.
3) Power Amplifiers: These amplifiers amplify the power of the input signal. This means
that the product of voltage and current at the output end is greater than the input
product. Only some part of the input AC voltage is amplified. Depending on the
portion of AC signal amplified (Schweber, 2017).
I. Class A: The collector current flows for the input signal’s negative and
positive half cycles. The circuit is shown in figure 7. The level of distortion is
very less. It is very stable, highly linear, the heat output is high during
operation. It is mainly used in audio circuits.

Figure 7 (Ravi, 2018)
II. Class B: The collector current flows for the input signal’s positive half cycle.
The circuit is shown in figure 8. It produces less heat during operation. It is
also stable and reliable, it has higher efficiency and it uses two transistor for
each cycle. It is used in push-pull amplification circuits.
Figure 8 (Ravi, 2018)
III. Class AB: The collector current flows less than the input signal’s full cycle but
more than the half cycle. The circuit is shown in figure 9. There is no cross-
over distortion, satisfactorily efficient, mixes the characteristics of A and B.

Figure 9 (Ravi, 2018)
IV. Class C: The collector current flows less than the input signal’s half cycle. The
circuit is shown in figure 10. The power dissipation is less, output distortion is
high along with efficiency. Due to high efficiency of collector, it is used to
amplify a narrow band of frequencies as a tuned amplifier (Analyse A Mater,
2016), (Ravi, 2018).
Figure 10 (Ravi, 2018)