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AF Amplifier based on NE592

The broadband amplifier NE592 can be used also for building a simple selective audio amplifier. By means of the +6 V and +12 V supply voltages a DC coupling between the product detector and the AF amplifier is possible. The DC level at the differential inputs is about 4,5 V. The mixer drives the differential input of the NE592 - after suppression of the RF proportions by two 47 nF capacitors - with the remaining AF signal. RF blocking between the outputs is feasible with only one capacitor, but not between the single outputs and ground. But the latter is absolutly necessary due to the wide amplifier bandwidth. The AF stage offers a gain of 200 (46 dB) at 660 Hz center frequency

Fig. 1: AF amplifier with NE592

Headphones with Ri > 1 kOhm are mandatory due to the relatively high output resistance of the NE592 amp. 18 mA quietscent current isn't low, but it is insignificant in case of stationary operation and supply by a small ac-operated power supply.

Fig . 2: Transfer characteristic

The selectivity isn't excelent, but better than none. Usual CW audio filters with e.g. four op amps need many times more components.

AF Amplifier based on TL072

You can reduce the AF stage current consumtion down to 2 mA when with a dual op amp TL072/082 instead of the NE592 broadband amp. These consumer op amps have a lower output resistance, so that they are able to drive 2 x 32 ohms headphones connected in series with a 470 ohms resistor. When using a NE5532 no additional resistor is necessary, because the outputs have a less lower impedance. The transfer characteristic is the same as the one displayed for the first circuit (AF amp with NE592)

Fig. 1: AF amplifier with a dual op amp TL072

AF Amplifier based on 2N3906/3904

Low distortion, CW selectivity, high gain, low noise and a high output voltage are the features of the following circuit. This circuit is seldom used despite its good characteristics. The latter is perhaps because of the fact that one must try different resistance values for R4 and R5 depending upon the quiescent current through IC1. This is ok for building a single device but not acceptable for a series product.

Fig. 1: 0,5 W audio amplifier

For low distortion an perfect bias adjustment of the transistors VT1/2 is necessary. One must define the resistance value of R4 and R5 in such a way that VT1 and VT2 get a base emitter voltage of approx. 500 mV dc with no drive signal. The series resonant circuit Dr1 and C3 is responsible for the CW selectivity. In case of resonance the stage has a gain of V  =  (1 - R8 / Rres)  = 60. The frequency transfer characteristic is identical to the one of the first circuit (AF amplifier based on NE592).

Fig. 2: Output signal

Contrary to the usual complementary stages with a common-collector arrangement, the output voltage attainable with this principle is almost equal the operating voltage (12 V - 2 x Uce). The good value acquires one with an increased output resistance, which is caused by the transistors operating in a common-emitter arrangement. The output resistance is optimal for headphones that have an impedance between 32 and 64 ohms. The current consumption (5 to 50 mA) depends upon the drive level and the load resistance.