Circuit Notebook 76 - Variable Gain Video Amplifier

Most ATV stations rely on a domestic satellite receiver for the reception of ATV transmissions on 24cms. Some satellite receivers work well without modification, but others require an RF pre-amplifier to increase the sensitivity and/or improve the front-end noise performance.

Also, as a result of the relatively narrow deviation of an ATV signal, compared with the wide deviation of a satellite transmission, many receivers produce a low video output signal, typically 0.5V peak-to-peak instead of the usual 1V p-p. In some satellite receivers it is possible to increase the video output signal by adjustment of an internal pre-set gain control, e.g. Maspro SRE 90R. Where this is not possible, an external video amplifier is required.

Video Amplifier Circuit

A suitable circuit using an Elantec EL2020CN 50 MHz current feedback op-amp (Farnell 399-115) is shown in Fig.1. It operates from a 'commonly available' single +12V supply. The circuit could be simplified if both + and - supplies were made available.

The overall gain of the circuit, video input to video output into a 75R load, is from X1 to X4 (0 to 12 dB), which depends on the setting of the pre-set gain control RV1. The measured -3 dB bandwidth is from 25 Hz to beyond 20 MHz. The prototype circuit was built on 'Veroboard' as shown in Fig.2.

Connection

Where the satellite receiver has a video output socket, this can be connected directly to the input of the video amplifier. Alternatively, where only a SCART socket is available then connection to pin 19 - video out and pin 17 - video earth/ground can be used. The pre-set control on the amplifier can be adjusted to provide a video output signal of 1V p-p into 75R, either by monitoring it with an oscilloscope, or by comparing it on a monitor with a known 1V p-p video signal, e.g. the output from a camcorder.

References

EL2020C Data Sheet, Elantec Semiconductor Inc. 1997 data Book, www.elantec.com/pages/

Figures

Fig.1.xx

Fig.2.xx