A swamped amplifier has a resistance tied to the emitter of the NPN transistor. Swamping the amplifier decreases the voltage gain. When an amplifier is swamped the voltage gain to the output is less dependent on the load. This helps to balance the output and protect the circuit when different loads might be applied. Fig. 10.37 shows the circuit of a swamped amplifier. Note that d.c. emitter resistance RE is divided into two parts viz. RE1 and RE2. Only resistance RE2 is bypassed by the capacitor CE while resistance RE1 is not. This method swamps or minimizes the effect of re′ on the voltage gain without reducing the voltage gain too much. Now the total a.c. emitter resistance is ( re′ + RE1) instead of re′ as in a standard CE amplifier. Therefore, the voltage gain of a swamped amplifier at no-load becomes :
If RE1 ≥ 10 re′ , then the effect of re′ is almost negligible and the voltage gain is given by ;
Av ~ Rc/Re
Therefore, the voltage gain is essentially independent of re′ or it is reasonably stabilised.
The Zin (base) with RE completely bypassed is Zin (base) = β re′ . When the emitter resistance is partially bypassed, the portion of the resistance that is unbypassed (i.e. RE1) is seen by the a.c. signal and appears in series with re′ . Therefore, for swamped amplifier,
Zin (base) = β ( re′ + RE1)
Example 10.21. Determine the value of voltage gain (Av) for the swamped amplifier shown in Fig. 10.38. What will be Zin (base) for this circuit?
Solution. In order to find voltage gain (Av), we first determine D.C. emitter current IE and then a.c. emitter resistance re′ . The value of IE can be determined as under :
Example 10.22. Determine the change in voltage gain for the amplifier in example 10.21 when re′ doubles in value.
Consequently, the change in Av is only 6.22% from the original value. In an amplifier that is not swamped, doubling the value of re′ would cause the value of Av to change (decrease) by *50%. Thus the voltage gain (Av) of the amplifier becomes more stable by swamping the emitter circuit.
Example 10.23. Fig. 10.39 shows the circuit of a **standard CE amplifier. The emitter circuit of this amplifier is swamped as shown in Fig. 10.40. Find :
(i) input impedance of transistor base [i.e. Zin (base)] for each circuit.
(ii) input impedance (Zin) for each circuit
Solution. Both the circuits have the same value of a.c. emitter resistance re′ . Therefore, following the standard procedure for finding re′ gives us a value of *25Ω for both circuits.
(i) Zin (base) For the standard CE amplifier shown in Fig. 10.39, we have,
Zin (base) = β re′ = 200 × 25Ω = 5 kΩ
For the swamped amplifier shown in Fig. 10.40, we have,
Zin (base) = β ( re′ + RE1)
= 200 (25Ω + 210 Ω) = 47000Ω = 47 kΩ
(ii) Zin For the standard CE amplifier shown in Fig. 10.39, we have,
Zin = R1 || R2 || Zin (base)
= 10 kΩ || 2.2 kΩ || 5 kΩ = 1.33 kΩ
For the swamped amplifier circuit shown in Fig. 10.40, we have,
Zin = R1 || R2 || Zin (base)
= 10 kΩ || 2.2. kΩ || 47 kΩ = 1.74 kΩ
Note that swamping increases the input impedance (Zin) of the amplifier. This reduces the amplifier’s loading effects on a previous stage.
Example 10.24. Find the voltage gain for both circuits of example 10.23. Solution. For the standard CE amplifier shown in Fig. 10.39, the voltage gain (Av) is given by ;
Av =Rc/re’=4k/25 ohm =160
For the swamped amplifier shown in Fig. 10.40, the voltage gain (Av) is given by ;
The following points may be noted ;
(i) The two circuits are identical for d.c. analysis purposes. Both have a total of 1.1. kΩ d.c. resistance in their emitter circuits.
(ii) For a standard CE amplifier, the total a.c. emitter resistance is re′ . When this amplifier is swamped, the total a.c. emitter resistance is increased to ( re′ + RE1).
(iii) Swamping reduces the voltage gain of the amplifier. However, the gain of a swamped amplifier is more stable than that of a comparable standard CE amplifier.
The transistor amplifiers may be classified as to their usage, frequency capabilities, coupling methods and mode of operation.
(i) According to use. The classifications of amplifiers as to usage are basically voltage amplifiers and power amplifiers. The former primarily increases the voltage level of the signal whereas the latter mainly increases the power level of the signal.
(ii) According to frequency capabilities. According to frequency capabilities, amplifiers are classified as audio amplifiers, radio frequency amplifiers etc. The former are used to amplify the signals lying in the audio range i.e. 20 Hz to 20 kHz whereas the latter are used to amplify signals having very high frequency.
(iii) According to coupling methods. The output from a single stage amplifier is usually insufficient to meet the practical requirements. Additional amplification is often necessary. To do this, the output of one stage is coupled to the next stage. Depending upon the coupling device used, the amplifiers are classified as R-C coupled amplifiers, transformer coupled amplifiers etc.
(iv) According to mode of operation. The amplifiers are frequently classified according to their mode of operation as class A, class B and class C amplifiers. This classification depends on the portion of the input signal cycle during which collector current is expected to flow. Thus, class A amplifier is one in which collector current flows for the entire a.c. signal. Class B amplifier is one in which collector current flows for half-cycle of input a.c. signal. Finally, class C amplifier is one in which collector current flows for less than half-cycle of a.c. signal.
Example 10.25. What do you understand by following amplifiers:
(i) Class A voltage amplifier (ii) Audio voltage amplifier (iii) Class B power amplifier (iv) Class A transformer coupled power amplifier ?
(i) Class A voltage amplifier means that it raises the voltage level of the signal and its mode of operation is such that collector current flows for the whole input signal.
(ii) Audio voltage amplifier means that it raises the voltage level of audio signal (i.e. one having frequency range 20 Hz to 20 kHz) and its mode of operation is class A.
(iii) It means that this amplifier raises the power level of the signal and its mode of operation is such that collector current flows for half-cycle of the signal only.
(iv) It means that power amplification is being done, coupling is by transformer and mode of operation is class A.
An amplifier can be replaced by an equivalent circuit for the purpose of analysis. Fig. 10.41 (i) shows the amplifier circuit while Fig. 10.41 (ii) shows its equivalent circuit.
V1 = input signal voltage to the amplifier
I1 = input signal current
Rin = input resistance of the amplifier
A0 = voltage gain of the amplifier when no load is connected
I2 = output current
V2 = output voltage across load RL
Rout = output resistance of the amplifier
RL = load resistance
Av = voltage gain when load RL is connected
Note that capability of the amplifier to produce voltage gain is represented by the voltage generator A0V1. The voltage gain of the loaded amplifier is Av. Clearly, Av will be less than A0 due to voltage drop in Rout.
If the signal source of voltage ES and resistance RS is considered, the amplifier equivalent circuit will be as shown in Fig. 10.42. Referring to Fig. 10.42, we have
Note. The use of such an equivalent circuit is restricted to the signal quantities only. Further, in drawing the equivalent circuit, it is assumed that the exact linear relationship exists between input and output signals i.e. the amplifier produces no waveform distortion.
Example 10.26. An amplifier has an open circuit voltage gain of 1000, an input resistance of 2 kΩ and an output resistance of 1Ω. Determine the input signal voltage required to produce an output signal current of 0.5A in 4Ω resistor connected across the output terminals.
Example 10.27. An amplifier has an open circuit voltage gain of 1000, an output resistance of 15Ω and an input resistance of 7kΩ. It is supplied from a signal source of e.m.f. 10mV and internal resistance 3kΩ. The amplifier feeds a load of 35 Ω. Determine (i) the magnitude of output voltage and (ii) power gain.
Example 10.28. An amplifier, when loaded by 2 kΩ resistor, has a voltage gain of 80 and a current gain of 120. Determine the necessary signal voltage and current to give an output voltage of 1V. What is the power gain of the amplifier?
We know that the process of raising the strength of an a.c. signal is called amplification and the circuit used to preform this function is called an amplifier. There are three types of gain : current gain, voltage gain and power gain.
(i) The common emitter (CE) amplifier exhibits all there types gain . From input to output, current will increase, voltage will increase and power will increase.
(ii) The common base (CB) amplifier has voltage gain and power gain but no current gain. Note that the current gain of a CB circuit is less than 1.
(iii) The common collector (CC) amplifier has current gain and power gain but no voltage gain. It is important to note that the type of gain an amplifier has depends upon the transistor configuration. Consequently, the choice of an amplifier for a given application often depends on the type of gain that is desired. Since CE arrangement is widely used (in about 90% applications), we shall be mainly concentrating on this type of circuit.
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Reference: Principles Of Electronics By V K Mehta And Rohit Mehta
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