Modulators and I-F Amplifiers [Transistor AF and RF Circuits (1965)]

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This is a four-transistor unit designed for use as an audio amplifier. It will supply from 10 to 12 watts of audio power, which is enough to provide 100% modulation for a final r-f amplifier where the final tube is a 6V6 or 2E26. A carbon-microphone input is shown, with the 12-volt source providing the power for the carbon mike into the input transformer. The 2N180 drives the 2N255, which in turn feeds the push-pull 2N256 output transistors.

Suggested transformer types are given for all three transformers; the output transformer can be of the universal tube-to-speaker type with several impedance taps. One of these impedance taps must be centered. One tap of a 16-ohm transformer goes to one collector, the common tap goes to the other collector, and the 8-ohm tap is connected to the negative end of the power supply. Because the transistor impedance is about 32 ohms as shown, the secondary impedance is twice the value that it is marked. For modulation, the secondary is connected in series with the B+ and the anode, or plate, of the final r-f amplifier.



1. Lower gain transistor, such as the ZN107, GTZZZ, and CK722, may be substituted for the ZN I 80 units.

2. TRIAD TY-54X, TZ-25, or THORDARSON TR-25.

3. TRIAD TY-61X, LAFAYETTE TR-85, ACME T-24042, N. E. 2552, ARGONNE AR504.

4. TRIAD TY-65Z modulation transformer, or tube-to-speaker output transformer matching 24 to 32: ohms input to 4, 000 ohms as described above.

5. Thermistor required to protect transistors when subjected to high temperature: VECO ZIWI (100 ohms, -3. 7.% /°C).

6. All resistors are ½ watt units except as noted.


All capacitors are in MFD.

T1 – 60 ohm to 440 C. T. T2 – 6 ohm C. T. to modulation impedance


This circuit shows a modulator of 25 watts, operating from a 12 volt supply. The modulator is suitable for use with a mobile transmitter, because of its light weight and small power drain. Two 2N1274 transistors are used as audio amplifiers, with the second transistor (02) feeding the driver (03), which is a type 2Nl038.

The driver is transformer-coupled to the push-pull output stage using 2N456A's. The secondary impedance of the modulating transformer is designed for the particular application for this modulator.

A 6-ohm center tap is necessary for the primary, while the secondary can be any desired impedance for the particular application.


This circuit shows the grown-junction tetrode-transistor 3N35 used as a 60-megacycle i-f amplifier. The single-stage gain is about 12 db at this frequency, using a 20-volt source as shown.


TYPICAL INTERSTAGE TRANSFORMER: 60-mhz i-f amplifier. Courtesy Texas Instruments Inc.


This circuit shows a four-transistor modulator that is essentially a high-power audio amplifier. Resistor R1 is used to limit the microphone current; resistor R2 biases transistors Q1 and Q2 collector current; resistors R6, R1, and R5 provide collector bias for transistors Q3 and Q4. It is necessary that transformer T2 have a center tap on the secondary, and this is obtained by unwinding 46 turns on the outside winding, bringing out the center tap, and rewinding the transformer.

This circuit requires very little current drain when it is not operating as a modulator without an input. The frequency response is essentially flat from 200 cycles to 7000 cycles.


R1-200 ohm, 2 watt, w.w. potentiometer

R2 -600 ohm, ½ watt, carbon resistor

R3-62K, ½ watt, carbon resistor

R4,5-47 ohm, ½ watt, carbon resistor

R6-0.1 ohm, 5 watt, w.w. resistor

R7-220 ohm, 1 watt, carbon resistor

R1-3.3 ohm, 1 watt, w.w. resistor

Q1, 2-Delco 2N3215, RCA 2N109, GE 2N43 or equivalent

Q3, 4-Delco 2N278 or Q5-501 power transistor

T1-Thordarson TR-5 or equiv.

T2-Thordarson TR-21 or equiv.

T3-Thordarson 21F10 or equiv.

M-SB carbon microphone

Courtesy Delco Radio Div., General Motors Corp. 25-watt modulator.




This circuit shows an eight-transistor modulator producing 100 watts output using a 25-volt power source. The low-level stages use type 2N1274 transistors, the driver uses a type 2N1038, while the output stage uses two 2N511 transistors. This modulator has a high input-impedance amplifier in the first two stages, a speech clipper in the second two stages, and a single driver stage with push-pull output.

In this circuit, as shown, the first five 2N1274 transistors are de signed to operate from a 12-volt supply, while the output stage requires 25 volts. With this arrangement it is possible to operate the speech amplifiers and the clipper directly from a 12-volt supply, if desired. Output from the modulation transformer is 100 watts, and the secondary impedance for this transformer may be selected for the desired application so that this modulator can be used with transmitters up to and including 200 watts of input power.


This circuit shows a two-transistor 12.5 -mhz i-f amplifier, using type 2N741 transistors. The overall power gain is 60 db, with a bandwidth of 200 khz and a current consumption of 9 milliamperes at 13.6 volts. This circuit does not have a tapped coil or mutual inductance coupling between the stages; instead, it uses a tapped capacitance to provide the necessary coupling. Input and output impedance, as shown, is 50 ohms.


This circuit shows a three-transistor 5.5-megacycle i-f amplifier using three type 2N2189 germanium transistors. With a generator impedance of 100 ohms and an output impedance of 400 ohms, this circuit has a power gain of 62 db, a bandwidth of 0.18 mhz, and a noise figure of 4 db.


This circuit shows the use of the transistor 2N741 in a 30 -mhz i-f amplifier. The input and output impedance to this amplifier are both 50 ohms, and a 13.6-volt supply is used. The coupling networks between stages do not have mutual inductance coupling or tapped coils, but a tuned circuit with a capacitive tap is used. Matching in this case is for necessary bandwidth and not for maximum power gain. This circuit provides a gain of 53 db and a bandwidth of 15 mhz, with a center frequency of 30 Mhz.


------------- Courtesy Motorola Semiconductor Products. 30 -mhz i-f amplifier.

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Updated: Monday, 2022-03-14 15:13 PST