Collector Modulation Circuit – Working, Advantages and Disadvantages:

This is very popular method for AM generation. The circuit diagram for collector modulation is shown in Fig. 22.26. Here, the transistor Q₁ makes an RF class C amplifier. The carrier signal is applied to the base of Q₁. V_CC makes collector supply used for biasing purpose. Also, the transistor Q₂ makes a class B amplifier which is used to amplify the audio or modulating signal. The modulating or baseband signal appears across the modulation transformer after amplification. The amplified modulating signal appears in series with collector supply V_CC. The function of capacitor C is to offer low impedance path to high frequency carrier signal and, therefore, the carrier signal is prevented from flowing through the modulation transformer.

Working Principle of Collector Modulation:

It is a known fact that in a class-C amplifier, the magnitude of output voltage is a definite fraction of or at the most equal to supply voltage V_CC. In addition to this, a linear relationship exists between the output tank current I_t and the variable supply voltage V_c (here it has been assumed that supply voltage V_CC is a varying quantity and its varying value is denoted by V_c). This means that in class C amplifier, the output voltage will be an exact replica of the input voltage waveform and the magnitude of output voltage will be approximately equal to carrier supply voltage V_CC. Now if R is the resistance of the output tank circuit at resonance, then the magnitude of output voltage is given as

RI_t ≡ V_CC

So, the unmodulated carrier is amplified by class C modulation amplifier using transistor Q₁ and its magnitude will remain constant at V_CC since there appears no voltage across the modulation transformer in the absence of modulating signal.

But now if a modulating voltage v_m = V_m cos ω_mt appears across the modulation transformer, this signal will be added to the carrier supply voltage V_CC. This results in a quite slow variation in carrier supply voltage V_CC. This type of slow variation in carrier supply voltage changes the magnitude of the carrier signal voltage and the output of the modulated class C amplifier as shown in Fig. 22.27.

It may be observed that the envelope of the output voltage is identical with the modulating voltage and thus AM signal is produced.

Mathematical Analysis:

The slowly varying carrier supply voltage V_c may be expressed as

Let the carrier voltage be v_C = V_CC cos ω_ct

Then the modulated output voltage will be

Substituting the value of V_c in above equation from Eq. (22.32), we have

which is required expression for AM wave.

Advantages:

1. Good linearity.
2. High collector circuit efficiency.
3. High power output per transistor.

Disadvantages:

1. Large modulating power requirement.
2. Hundred per cent modulation cannot be achieved because of collector saturation.