دارة أخرى لمذبذب مانع مع مولد سن المنشار (ترانزستوري)

14 - 2 other circuits for a blocking oscillator with a sawtooth generator (transistor)

This circuit is similar in its work to the valve circuits of the barrier oscillator - where the sawtooth works to charge the capacitor C1 during the period of the upward line and discharge the capacitor during the downward line. The load on the capacitor C1 is zero and cuts the transistor Q1 by the effect of the reverse bias voltage across R7 which is applied to the emitter. At first the base is ground voltage, the rising line starts at time T1 when the capacitor C1 charges through the resistance 4 in the direction, and when the voltage increases on C1 in the negative polarity - Part of it feeds the Q1 base via the T1 secondary. It is a forward bias voltage from the divider R1 and R2, and the time T2 represents the end of the scan. Like a forward bias. R2 - At this moment, the transistor Q1 is connected - and the collector current passing through the transformer T1 causes the base voltage to fluctuate, mostly with negative polarity - and the two voltages gather in series, namely the voltage in the resistance R2 and the secondary voltage of the transformer T1. They lead transistor Q1 towards saturation, to start the oscillating part of the sawtooth (which is the downward line), and the transistor and transformer primary T1 and R7 form a low resistance passing the discharge current of the capacitor C1. At the end of this rapid discharge, its discharge reaches almost zero.

Transistor Q1 is cut again by the reverse bias. This completes the Retrace line.

The frequency is controlled by changing the amount of reverse bias of the Q1 emitter, and by increasing the negative voltage by adjusting the resistor R6 the time of 01 in the cut-off state increases - this increases the charging time of the capacitor, which actually lowers the frequency of the oscillator. The resistance of 6 (100) ohms performs the job of suppressing resistance across the primary. T1 to reduce the return transients to safe limits. The collector-base connection requires this because a possible voltage breakdown may occur, transistor damage causing Q1

14-3 Drivers (Motivation):

​​​​​​The sawtooth wave is generated in the valve circuit by allowing the capacitor to charge to a part of the applied voltage - then the voltage of this capacitor is fed to the output amplifier valve network - and the large impedance of the valve entrance is not a load on the capacitor.

But in the transistor circuit, a wave from the saw is fed into the next stage transistor base circuit. This is a load on the capacitor because its impedance is small, not exceeding several hundred ohms - this small resistance leads to a relatively large current draw, which reduces the amplitude of the sawtooth voltage and makes it non-linear

The solution to the problem of loading the capacitor formed by the saw tooth and to increase the current gain from the saw to operate the amplifier (output) in feeding the saw tooth to an operating amplifier, this amplifier must have a large impedance in its entrance connected to a capacitor formed from the saw in order to prevent its loading - and this amplifier has a low impedance in its output to match the input of the next amplifier. The common collector amplifier method (radiator follower) can be used to achieve this in Figure (3-14).

The direct connection method is used at the output of the amplifier, and this is what limits the necessity of using a large-capacity connection capacitor with a greater cost.

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