What Is RC Coupled Amplifier?
This is the most popular type of coupling because it is cheap and provides excellent audio fidelity over a wide range of frequency. It is usually employed for voltage amplification. Fig. 11.9 shows two stages of an RC coupled amplifier. A coupling capacitor CC is used to connect the output of first stage to the base (i.e. input) of the second stage and so on. As the coupling from one stage to next is achieved by a coupling capacitor followed by a connection to a shunt resistor, therefore, such amplifiers are called resistance capacitance coupled amplifiers.
The resistances R1, R2 and RE form the biasing and stabilisation network. The emitter bypass capacitor offers low reactance path to the signal. Without it, the voltage gain of each stage would be lost. The coupling capacitor CC transmits a.c. signal but blocks d.c. This prevents d.c. interference between various stages and the shifting of operating point.
Operation Of RC Coupled Transistor Amplifier
When a.c. signal is applied to the base of the first transistor, it appears in the amplified form across its collector load RC. The amplified signal developed across RC is given to base of next stage through coupling capacitor CC. The second stage does further amplification of the signal. In this way, the cascaded (one after another) stages amplify the signal and the overall gain is considerably increased. It may be mentioned here that total gain is less than the product of the gains of individual stages. It is because when a second stage is made to follow the first stage, the effective load resistance of first stage is reduced due to the shunting effect of the input resistance of second stage. This reduces the gain of the stage which is loaded by the next stage. For instance, in a 3-stage amplifier, the gain of first and second stages will be reduced due to loading effect of next stage. However, the gain of the third stage which has no loading effect of subsequent stage, remains unchanged. The overall gain shall be equal to the product of the gains of three stages.
Frequency responseRC Coupled Transistor Amplifier
. Fig.11.10 shows the frequency response of a typical RC coupled amplifier. It is clear that voltage gain drops off at low (< 50 Hz) and high (> 20 kHz) frequencies whereas it is uniform over mid-frequency range (50 Hz to 20 kHz). This behaviour of the amplifier is briefly explained below :
At low frequencies (< 50 Hz), the reactance of coupling capacitor CC is quite high and hence very small part of signal will pass from one stage to the next stage. Moreover, CE cannot shunt the emitter resistance RE effectively because of its large reactance at low frequencies. These two factors cause a falling of voltage gain at low frequencies.
At high frequencies (> 20 kHz), the reactance of CC is very small and it behaves as a short circuit. This increases the loading effect of next stage and serves to reduce the voltage gain. Moreover, at high frequency, capacities reactance of base-emitter junction is low which increases the base current. This reduces the current amplification factor β. Due to these two reasons, the voltage gain drops off at high frequency.
At mid-frequencies (50 Hz to 20 kHz), the voltage gain of the amplifier is constant. The effect of coupling capacitor in this frequency range is such so as to maintain a uniform voltage gain. Thus, as the frequency increases in this range, reactance of CC decreases which tends to increase the gain. However, at the same time, lower reactance means higher loading of first stage and hence lower gain. These two factors almost cancel each other, resulting in a uniform gain at mid-frequency.
- It has excellent frequency response. The gain is constant over the audio frequency range which is the region of most importance for speech, music etc.
- It has lower cost since it employs resistors and capacitors which are cheap.
- The circuit is very compact as the modern resistors and capacitors are small and extremely light.
- The RC coupled amplifiers have low voltage and power gain. It is because the low resistance presented by the input of each stage to the preceding stage decreases the effective load resistance (RAC) and hence the gain.
- They have the tendency to become noisy with age, particularly in moist climates.
- Impedance matching is poor. It is because the output impedance of RC coupled amplifier isseveral hundred ohms whereas the input impedance of a speaker is only a few ohms. Hence, little power will be transferred to the speaker.
Applications RC Coupled Transistor Amplifier
The RC coupled amplifiers have excellent audio fidelity over a wide range of frequency. Therefore, they are widely used as voltage amplifiers e.g. in the initial stages of public address system. If other type of coupling (e.g. transformer coupling) is employed in the initial stages, this results in frequency distortion which may be amplified in next stages. However, because of poor impedance matching, RC coupling is rarely used in the final stages.
Note. When there is an even number of cascaded stages (2, 4, 6 etc), the output signal is not inverted from the input. When the number of stages is odd (1, 3, 5 etc.), the output signal is inverted from the input.
Example 11.11 A single stage amplifier has a voltage gain of 60. The collector load RC = 500 Ω and the input impedance is 1kΩ. Calculate the overall gain when two such stages are cascaded through R-C coupling. Comment on the result.
Solution. The gain of second stage remains 60 because it has no loading effect of any stage. However, the gain of first stage is less than 60 due to the loading effect of the input impedance of second stage.
Comments. The gain of individual stage is 60. But when two stages are coupled, the gain is not 60 × 60 = 3600 as might be expected rather it is less and is equal to 2397 in this case. It is because the first stage has a loading effect of the input impedance of second stage and consequently its gain is reduced. However, the second stage has no loading effect of any subsequent stage. Hence, the gain of second stage remains 60.
Example 11.12. Fig. 11.11 shows two-stage RC coupled amplifier. If the input resistance Rin of each stage is 1kΩ, find : (i) voltage gain of first stage (ii) voltage gain of second stage (iii) total voltage gain
Example 11.13. A single stage amplifier has collector load RC = 10 kΩ; input resistance Rin = 1kΩ and β = 100. If load RL = 100Ω, find the voltage gain. Comment on the result.
Comments. As the load (e.g. speaker) is only of 100 ohms, therefore, effective load of the amplifier is too much reduced. Consequently, voltage gain is quite small. Under such situations, we can use a transformer to improve the voltage gain and signal handling capability. For example, if the output to 100 Ω load is delivered through a step-down transformer, the effective collector load and hence voltage gain can be increased.
Example 11.14. Fig. 11.12 shows a 2-stage RC coupled amplifier. What is the biasing potential for the second stage ? If the coupling capacitor CC is replaced by a wire, what would happen to the circuit ?
Thus biasing potential for the second stage is 3.6 V. When the coupling capacitor CC is replaced by a wire, this changes the entire picture. It is because now RC of the first stage is in parallel with R3 of the second stage as shown in Fig. 11.13(i). The total resistance of RC (= 3.6 kΩ) and R3 (= 10 kΩ) is given by:
Thus the biasing potential of second stage is drastically changed. The 9.07 V at the base of Q2 would undoubtedly cause the transistor to saturate and the device would be rendered useless as an amplifier. This example explains the importance of dc isolation in a multistage amplifier. The use of coupling capacitor allows each amplifier stage to maintain its independent biasing potential while allowing the ac output from one stage to pass on to the next stage.
Example 11.15. Fig. 11.14 shows a 2-stage RC coupled amplifier. Find the voltage gain of (i) first stage (ii) second stage and (iii) overall voltage gain.
Solution. (i) Voltage gain of First stage. The input impedance of the second stage is the load for the first stage. In order to find input impedance of second stage, we shall first find r′e (ac emitter resistance) for the second stage.
(ii) Voltage gain of second stage. The load RL (= 10 kΩ) is the load for the second stage.
∴ Effective collector load for second stage is
(iii) Overall voltage gain. Overall voltage gain = First stage gain × Second stage gain
= 53 × 191.4 = 10144
Transformer Coupled Amplifier
The main reason for low voltage and power gain of RC coupled amplifier is that the effective load (RAC) of each stage is *decreased due to the low resistance presented by the input of each stage to the preceding stage. If the effective load resistance of each stage could be increased, the voltage and power gain could be increased. This can be achieved by transformer coupling. By the use of **im-pedance-changing properties of transformer, the low resistance of a stage (or load) can be reflected as a high load resistance to the previous stage. Transformer coupling is generally employed when the load is small. It is mostly used for power amplification. Fig. 11.15 shows two stages of transformer coupled amplifier. A coupling transformer is used to feed the output of one stage to the input of the next stage. The primary P of this transformer is made the collector load and its secondary S gives input to the next stage.
Operation Of Transformer Coupled Amplifier
When an a.c. signal is applied to the base of first transistor, it appears in the amplified form across primary P of the coupling transformer. The voltage developed across primary is transferred to the input of the next stage by the transformer secondary as shown in Fig.11.15. The second stage renders amplification in an exactly similar manner.
Frequency response Transformer Coupled Amplifier
The frequency response of a transformer coupled amplifier is shown in Fig.11.16. It is clear that frequency response is rather poor i.e. gain is constant only over a small range of frequency. The output voltage is equal to the collector current multiplied by reactance of primary. At low frequencies, the reactance of primary begins to fall, resulting in decreased gain. At high frequencies, the capacitance between turns of windings acts as a bypass condenser to reduce the output voltage and hence gain. It follows, therefore, that there will be disproportionate amplification of frequencies in a complete signal such as music, speech etc. Hence, transformer-coupled amplifier introduces frequency distortion. It may be added here that in a properly designed transformer, it is possible to achieve a fairly constant gain over the audio frequency range. But a transformer that achieves a frequency response comparable to RC coupling may cost 10 to 20 times as much as the inexpensive RC coupled amplifier.
Advantages Of Transformer Coupled Amplifier
- No signal power is lost in the collector or base resistors.
- An excellent impedance matching can be achieved in a transformer coupled amplifier. It is easy to make the inductive reactance of primary equal to the output impedance of the transistor and inductive reactance of secondary equal to the input impedance of next stage.
- Due to excellent impedance matching, transformer coupling provides higher gain. As a matter of fact, a single stage of properly designed transformer coupling can provide the gain of two stages of RC coupling.
Disadvantages Of Transformer Coupled Amplifier
- It has a poor frequency response i.e.the gain varies considerably with frequency.
- The coupling transformers are bulky and fairly expensive at audio frequencies.
- Frequency distortion is higher i.e. low frequency signals are less amplified as compared to the high frequency signals.
- Transformer coupling tends to introduce *hum in the output.
Applications Of Transformer Coupled Amplifier
Transformer coupling is mostly employed for impedance matching. In general, the last stage of a multistage amplifier is the power stage. Here, a concentrated effort is made to transfer maximum power to the output device e.g. a loudspeaker. For maximum power transfer, the impedance of power source should be equal to that of load. Usually, the impedance of an output device is a few ohms whereas the output impedance of transistor is several hundred times this value. In order to match the impedance, a step-down transformer of proper turn ratio is used. The impedance of secondary of the transformer is made equal to the load impedance and primary impedance equal to the output impedance of transistor. Fig. 11.17 illustrates the impedance matching by a stepdown transformer. The output device (e.g. speaker) connected to the secondary has a small resistance RL. The load R′ L appearing on the primary side will be:
Thus the load on the primary side is comparable to the output impedance of the transistor. This results in maximum power transfer from transistor to the primary of the transformer. This shows that low value of load resistance (e.g. speaker) can be “stepped-up” to a more favorable value at the collector of transistor by using appropriate turn ratio.
Example 11.16. A transformer coupling is used in the final stage of a multistage amplifier. If the output impedance of transistor is 1kΩ and the speaker has a resistance of 10Ω, find the turn ratio of the transformer so that maximum power is transferred to the load.
Solution. For maximum power transfer, the impedance of the primary should be equal to the output impedance of transistor and impedance of secondary should be equal to load impedance i.e.
Example 11.17. Determine the necessary transformer turn ratio for transferring maximum power to a 16Ω load from a source that has an output impedance of 10 kΩ. Also calculate the voltage across the external load if the terminal voltage of the source is 10V r.m.s.
For maximum power transfer, the impedance of the primary should be equal to the output impedance of the source.
Example 11.18. The output resistance of the transistor shown in Fig. 11.18 is 3kΩ. The primary of the transformer has a d.c. resistance of 300 Ω and the load connected across secondary is 3Ω. Calculate the turn ratio of the transformer for transferring maximum power to the load.
D.C. resistance of primary, RP = 300 Ω
Load resistance, RL = 3 Ω
Let n ( = NP /NS) be the required turn ratio. When no signal is applied, the transistor ‘sees’ a load of RP (= 300 Ω) only. However, when a.c. signal is applied, the load RL in the secondary is reflected in the primary as n 2 RL. Consequently, the transistor now ‘sees’ a load of RP in series with n 2 RL. For transference of maximum power
Example 11.19. A transistor uses transformer coupling for amplification. The output impedance of transistor is 10 kΩ while the input impedance of next stage is 2.5 kΩ. Determine the inductance of primary and secondary of the transformer for perfect impedance matching at a frequency of 200 Hz.
Example 11.20. In the above example, find the number of primary and secondary turns. Given that core section of the transformer is such that 1 turn gives an inductance of 10μH.
Solution. We know that inductance of a coil is directly proportional to the square of number of turns of the coil i.e
Direct Coupled Amplifier
There are many applications in which extremely low frequency (< 10 Hz) signals are to be amplified e.g. amplifying photo-electric current, thermo-couple current etc. The coupling devices such as capacitors and transformers cannot be used because the electrical sizes of these components become very large at extremely low frequencies. Under such situations, one stage is directly connected to the next stage without any intervening coupling device. This type of coupling is known as direct coupling.
Circuit details. Fig. 11.19 shows the circuit of a three-stage direct-coupled amplifier. It uses *complementary transistors. Thus, the first stage uses npn transistor, the second stage uses pnp transistor and so on. This arrangement makes the design very simple. The output from the collector of first transistor T1 is fed to the input of the second transistor T2 and so on.
The weak signal is applied to the input of first transistor T1. Due to transistor action, an amplified output is obtained across the collector load RC of transistor T1. This voltage drives the base of the second transistor and amplified output is obtained across its collector load. In this way, direct coupled amplifier raises the strength of weak signal.
Advantages Of Direct Coupled Amplifier
- The circuit arrangement is simple because of minimum use of resistors.
- The circuit has low cost because of the absence of expensive coupling devices.
Disadvantages Of Direct Coupled Amplifier
- It cannot be used for amplifying high frequencies.
- The operating point is shifted due to temperature variations.
Example 11.21. Fig. 11.20 shows a direct coupled two-stage amplifier. Determine (i) d.c. voltages for both stages (ii) voltage gain of each stage and overall voltage gain.
Solution. Note that direct-coupled amplifier has no coupling capacitors between the stages.
(i) D.C. voltages. We shall now determine the d.c. voltages for both the stages following the established procedure.
(ii) Voltage gain To find voltage gain, we shall use the standard formula : total a.c. collector load
divided by total a.c. emitter resistance.
Comparison of Different Types of Coupling
|S. No||Particular||RC coupling||Transformer coupling||Direct coupling|
|1||Frequency response||Excellent in the audio |
|3||Space and weight||Less||More||Least|
|4||Impedance matching||Not good||Excellent||Good|
|5||Use||For voltage amplification||For power amplification||For amplifying extremely low frequencies|
Difference Between Transistor and Tube Amplifiers
Although both transistors and grid-controlled tubes (e.g. triode, tetrode and pentode) can render the job of amplification, they differ in the following respects :
- The electron tube is a voltage-driven device while transistor is a current operated device.
- The input and output impedances of the electron tubes are generally quite large. On the other hand, input and output impedances of transistors are relatively small.
- Voltages for transistor amplifiers are much smaller than those of tube amplifiers.
- Resistances of the components of a transistor amplifier are generally smaller than the resistances of the corresponding components of the tube amplifier.
- The capacitances of the components of a transistor amplifier are usually larger than the corresponding components of the tube amplifier
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