Supply Voltage Normalj

g2 2K re typical i oh 2w final g2 2K re the base of Qx gives full modulation. This drive can be obtained from one or two stages of common emitter amplification depending upon the microphone used, can be almost any type oi good quality pnp transistor.

Performance wise, the dc-series technique is superior to the conventional method since it yields a greater modulated power output, lower distortion, and overall transmitter current is lower. Moreover, in a comparison test a larger detected audio voltage was achieved (with a diode demodulator) from the dc-series modulated transmitter than from the same final when it was transformer coupled collector modulated. This is an important criteria considering that what really counts, in an AM system, is the detected audio.

RC-cqupIed base modulation

A second circuit, RC-coupled base modulation, shown in Fig. 2, also has performance capabilities that are competitive with the usual collector modulation technique.

In this circuit, the modulation is injected to the base of the RF transistor using two resistors, RI and R2. The effect of IU is to linearize the waveform which is excellent for val-lues of R1 between 10 and 20 ohms. Negligible improvement in linearis is achieved for larger values of RL Also, R1 should not be bypassed for audio because bypassing introduces negative current feedback in the final RF stage at audio frequencies. Resistor R2 can range between 100 and 2>000 ohms, Ultimately, the upper value of both R1 and R2 is determined by the available rf drive power because greater drive power is needed for larger values of resistance.

The effective load resistance, presented to Qi, is essentially equal to R2. Therefore, to reduce modulation power requirements it is desirable to make R2 large- Thus, with these conflicting requirements some type of a compromise is necessary. Hence, tlie values given in Fig, 2 represent a good compromise between linearity, rf drive power and modulation power requirements.

Fig. 2, RC coupled base modulation.

i his re-coupled base circuit yields excellent modulation characteristics because it has built-in feedback, and. it also prevents feed through which in turn permits 100% modulation to be easily achieved. A disadvantage is the higher power dissipation in ihe rf stage due to the extra drive discussed previously. Therefore, you may want to add a little additional heat sinking to take care of the additional power dissipation.

The audio stage QA uses a small-signal, general purpose audio transistor in a common emitter stage. From the circuit components shown, an undistorted output of about 2 volts can be supplied to the base ol the final with 40 mV of audio drive signal. This is sufficient to provide 100$ modulation for a transmitter in the 1 to 2 watt range. For higher power finals, an audio stage capable of supplying a greater voltage swing should be used. Since the audio signal is being applied to Ihe base of the rf stage, it functions as a common emitter amplifier for both the RF and audio.

Wh\ not try dc-series modulation or rc-coupled base modulation next time—they both perform well and do away with the bulky, expensive transformer that is difficult to match.

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