Oscilloscope & Counter Preamplifier Design of Video Amplifier

A circuit containing a single NE592A video amplifier (the only active component) can be used to increase the sensitivity of an older oscilloscope or frequency counter.

Figure 4-5. Oscilloscope/Counter Preamplifier Circuit

Figure 4-5 shows a circuit which will provide a 20 ± 0.1 dB voltage gain from 500 kHz to 50 MHz. The low-frequency response of the amplifier may be extended by increasing the value of the 0.05µF capacitor connected in series with the input terminal. This circuit will yield an input-noise level of approximately 10µV over a 15.7 MHz bandwidth.

 

The gain can be calibrated by adjusting the potentiometer connected between pins 4 and 11 (gain adjust terminals). These pins go directly to the emitter terminals of the two npn differential input amplifiers. The 1000Ω potentiometer (a cermet type trimmer) can be adjusted for an exact voltage gain of 10. This preserves the scale factor of the instrument. The usual precautions of short leads and wide area ground planes for low-inductance ground systems, should result in good high-frequency response. A compact assembly package for an oscilloscope/counter preamplifier can be made by forming a small piece of sheet copper or brass into a the shape of a U.