Transformer Coupled Amplifier and its Frequency Response

Transformer – Coupled Amplifier

The main drawback of RC coupled amplifier is low voltage and power gains and poor impedance matching. This is because of the decrease in effective load R_AC of each stage. If the effective load resistance of each stage could be increased, the voltage and power gain could be increased. This van be achieved by transformer coupling. Transformer coupling is generally employed when the load is small. It is mostly used for power amplification.

Transformer Coupled Amplifier 

The above fig shows the two stage transformer coupled amplifier in this circuit the primary of transformer is placed instead of R_C. The secondary of transformer replaces a wire between the voltage divider network and base of second stage. The resistors R₁, R₂, and R_E forms the voltage divider bias and stabilization resistors. The capacitor C_E bypass the resistor R_E.

Operation: The input signal is applied to the base of the first transistor through the capacitor. The amplified current flows through the primary of transformer. The induced secondary voltage is applied to the base of the second transistor. Since the transformer is efficient in providing the d.c. isolation, the biasing of second stage is not disturbed. In addition to d.c. isolation transformer provides the proper impedance matching. The output is taken across the secondary of transformer.

Frequency Response of Transformer Coupled Amplifier

The frequency response of a transformer coupled amplifier as shown in below fig. it can be seen from the fig that the gain is not constant for particular band of frequencies. The 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 ants as a bypass capacitor o 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.

Frequency Response of Transformer Coupled Amplifier