Laboratory Amplifier For Audio And Ultrasonic Frequencies

The Type 1206-B Unit Amplifier, newest member of the General Radio family of Unit Instruments,1 is a high-quality amplifier, designed for general laboratory use. Its maximum output power of 3 watts is adequate for driving low-power transducers, and its wide frequency range (up to 250 kc) makes it useful as an amplifier for the output of such oscillators as the Type 1301-A and the Type 1302-A or for pulses with rise times as short as one microsecond. In conjunction with a pair of earphones, it has sufficient gain for use as a null detector in bridge measurements.

The General Radio unit construction makes possible a low price and a small compact assembly, which can be bolted to the Type 1203-A Unit Power Supply to form a single unit.

Circuit Details

This amplifier takes advantage of the desirable characteristics of the single-ended push-pull circuit to obtain low distortion over a wide frequency range.

•For other Unit Instruments, see the General Radio Experimenter for May, 1950; July, 1951; February, 1952; February, 1953, and September, 1953.

This type of circuit has been discussed in previous articles2,3 and is shown in basic form in Figure 1. Note that the upper output tube is driven from grid to cathode, not grid to ground, and that the a-c currents of both tubes add in the load, while the grid voltages are of opposite phase. Therefore it has true push-pull operation and single-ended output without the use of an output transformer. In a conventional push-pull circuit, the frequency range is severely limited by the output transformer.

The unit amplifier was designed for use with the Type 1203-A Unit Power

2A. P. G. Peterson, "A New Push-Pull Amplifier Circuit," General Radio Experimenter, October, 1951, pp. 1-7. 3A. P. G. Peterson and 1). B. Sinclair, "A Single-Ended Push-Pull Amplifier." Proc. I.R.E., January, 1952, pp. 7-11.

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