Transistors
- Bias Compensation
- Methods of Transistor Biasing
- Transistor Biasing
- Transistor as an Amplifier
- Operating Point
- Transistor Load Line Analysis
- Transistor Regions of Operation
- Transistor Configurations
- Transistors - Overview
Amplifiers
- Noise in Amplifiers
- Emitter Follower & Darlington Amplifier
- Negative Feedback Amplifiers
- Feedback Amplifiers
- Types of Tuned Amplifiers
- Tuned Amplifiers
- Class AB and C Power Amplifiers
- Class B Power Amplifier
- Push-Pull Class A Power Amplifier
- Transformer Coupled Class A Power Amplifier
- Class A Power Amplifiers
- Classification of Power Amplifiers
- Power Amplifiers
- Direct Coupled Amplifier
- Transformer Coupled Amplifier
- RC Coupling Amplifier
- Multi-Stage Transistor Amplifier
- Based on Configurations
- Classification of Amplifiers
- Basic Amplifier
Amplifiers Useful Resources
Selected Reading
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- HR Interview Questions
- Effective Resume Writing
- Questions and Answers
- UPSC IAS Exams Notes
Transformer Coupled Class A Power Amppfier
The class A power amppfier as discussed in the previous chapter, is the circuit in which the output current flows for the entire cycle of the AC input supply. We also have learnt about the disadvantages it has such as low output power and efficiency. In order to minimize those effects, the transformer coupled class A power amppfier has been introduced.
The construction of class A power amppfier can be understood with the help of below figure. This is similar to the normal amppfier circuit but connected with a transformer in the collector load.
![Divider Arrangement](/amppfiers/images/spanider_arrangement.jpg)
Here R1 and R2 provide potential spanider arrangement. The resistor Re provides stabipzation, Ce is the bypass capacitor and Re to prevent a.c. voltage. The transformer used here is a step-down transformer.
The high impedance primary of the transformer is connected to the high impedance collector circuit. The low impedance secondary is connected to the load (generally loud speaker).
Transformer Action
The transformer used in the collector circuit is for impedance matching. RL is the load connected in the secondary of a transformer. RL’ is the reflected load in the primary of the transformer.
The number of turns in the primary are n1 and the secondary are n2. Let V1 and V2 be the primary and secondary voltages and I1 and I2 be the primary and secondary currents respectively. The below figure shows the transformer clearly.
![Step Down](/amppfiers/images/step_down.jpg)
We know that
$$frac{V_1}{V_2} = frac{n_1}{n_2}: and: frac{I_1}{I_2} = frac{n_1}{n_2}$$
Or
$$V_1 = frac{n_1}{n_2}V_2 : and: I_1 = frac{n_1}{n_2}I_2$$
Hence
$$frac{V_1}{I_1} = left ( frac{n_1}{n_2} ight )^2 frac{V_2}{I_2}$$
But V1/I1 = RL’ = effective input resistance
And V2/I2 = RL = effective output resistance
Therefore,
$$R_L’ = left ( frac{n_1}{n_2} ight )^2 R_L = n^2 R_L$$
Where
$$n = frac{number : of : turns : in : primary}{number: of: turns: in: secondary} = frac{n_1}{n_2}$$
A power amppfier may be matched by taking proper turn ratio in step down transformer.
Circuit Operation
If the peak value of the collector current due to signal is equal to zero signal collector current, then the maximum a.c. power output is obtained. So, in order to achieve complete amppfication, the operating point should pe at the center of the load pne.
The operating point obviously varies when the signal is appped. The collector voltage varies in opposite phase to the collector current. The variation of collector voltage appears across the primary of the transformer.
Circuit Analysis
The power loss in the primary is assumed to be negpgible, as its resistance is very small.
The input power under dc condition will be
$$(P_{in})_{dc} = (P_{tr})_{dc} = V_{CC} imes (I_C)_Q$$
Under maximum capacity of class A amppfier, voltage swings from (Vce)max to zero and current from (Ic)max to zero.
Hence
$$V_{rms} = frac{1}{sqrt{2}} left [frac{(V_{ce})_{max} - (V_{ce})_{min}}{2} ight ] = frac{1}{sqrt{2}} left[ frac{(V_{ce})_{max}}{2} ight ] = frac{2V_{CC}}{2sqrt{2}} = frac{V_{CC}}{sqrt{2}}$$
$$I_{rms} = frac{1}{sqrt{2}} left [frac{(I_C)_{max} - (I_C)_{min}}{2} ight ] = frac{1}{sqrt{2}} left[ frac{(I_C)_{max}}{2} ight ] = frac{2(I_C)_Q}{2sqrt{2}} = frac{(I_C)_Q}{sqrt{2}}$$
Therefore,
$$(P_O)_{ac} = V_{rms} imes I_{rms} = frac{V_{CC}}{sqrt{2}} imes frac{(I_C)_Q}{sqrt{2}} = frac{V_{CC} imes (I_C)_Q}{2}$$
Therefore,
Collector Efficiency = $frac{(P_O)_{ac}}{(P_{tr})_{dc}}$
Or,
$$(eta)_{collector} = frac{V_{CC} imes (I_C)_Q}{2 imes V_{CC} imes (I_C)_Q} = frac{1}{2}$$
$$= frac{1}{2} imes 100 = 50\%$$
The efficiency of a class A power amppfier is nearly than 30% whereas it has got improved to 50% by using the transformer coupled class A power amppfier.
Advantages
The advantages of transformer coupled class A power amppfier are as follows.
No loss of signal power in the base or collector resistors.
Excellent impedance matching is achieved.
Gain is high.
DC isolation is provided.
Disadvantages
The disadvantages of transformer coupled class A power amppfier are as follows.
Low frequency signals are less amppfied comparatively.
Hum noise is introduced by transformers.
Transformers are bulky and costly.
Poor frequency response.
Apppcations
The apppcations of transformer coupled class A power amppfier are as follows.
This circuit is where impedance matching is the main criterion.
These are used as driver amppfiers and sometimes as output amppfiers.