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Typical R.F. Class A Amplifier

Author: J.B. Hoag

Fig. 27 A. A typical tuned radio-frequency amplifier

L1 C1 = the tuned-grid circuitL3= input primary coil
R1 = cathode resistorL1= input secondary coil
C2 = cathode bypass condenserC1 C5 = tuning condensers
C3 = screen bypass condenserC4 = plate bypass condenser
R2 = screen dropping resistorL4 = output secondary coil
L3 = primary of output transformerR4 = bleeder resistor
L4 C5 = second-stage tuned-grid circuitR3 = gain control resistor
----- = metal shields

Figure 27 A shows the type of r.f. amplifier commonly used in receiver circuits. The input and output circuits L1C1 and L4C5 are tuned to the same frequency.

The circuit would oscillate if a triode were used, because of the large feedback through the internal capacities of the tube itself; hence a pentode is used, with its internal screening action. In addition, it is necessary to prevent external feedback from the plate circuit to the grid by means of grounded metal shields, indicated by the dotted lines in Fig. 27 A, and by keeping the plate and grid lead wires as far apart as possible. The shield decreases the inductance and increases the resistance, i.e., changes the tuning, and decreases the Q of the coil, depending upon the shield material and its distance from the coil. Tuning of the circuits must therefore be done while the shield is in place. Also, to prevent oscillation, connect C3 and C4 to the ground or cathode with separate wires, each of which is as short as possible.

Bypass condensers C2, C3, and C4 should have low reactance at the frequencies to be amplified; 0.01 μfd. for ordinary communication purposes. The cathode resistor R1 should establish the minimum bias voltage recommended for the particular tube used. In applying Ohm's law to calculate R1 do not forget that both the screen and plate currents flow through R1. Ohm's law is also used to calculate R2, such that the voltage drop in it due to the screen current, subtracted from the B voltage, leaves the correct screen voltage. The tuned circuits should have a fairly high L/C ratio. L3 is usually coupled as closely to L4 as possible and contains 70 to 80 per cent of the number of turns on L4.

With variable-μ tubes, the gain control is accomplished by changing the C-bias voltage. There are three currents passing through the control resistor R3, i.e., the bleeder current through R4, the screen, and the plate current. The total drop across R3 should be about 50 volts. Maximum gain occurs when the sliding contact on R3 is at the grounded end. In multistage amplifiers, the lower ends of the various cathode resistors are sometimes connected together and then to the sliding contact on R3. The left end of R3 must then have a current-carrying capacity sufficient for all the tube currents plus that of the bleeder R4.

Last Update: 2010-11-27