Variable gain, zero offset, dc coupled video amplifier for microwave detector diodes

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Abstract
A variable gain, zero offset, dc coupled video amplifier has been developed which uses a recently introduced ±5 V operational amplifier with 1.6 GHz gain‐bandwidth product. The amplifier is inexpensive and it is particularly suited for amplifying the outputs of microwave diode detectors and driving 50 Ω loads. The large input bias currents of wideband operational amplifiers increase the sensitivity of positive polarity diode detectors, but they decrease the sensitivity when a negative polarity diode detector is used at the amplifier input. To eliminate this effect, matched N‐channel junction field‐effect transistors (JFETs) are used to buffer the operational amplifier inputs, which reduces the input bias current to ∼1 pA. An added benefit of the JFETs is that the amplifier offset voltage is unaffected by changes in gain or source impedance. Once the offset is zeroed out, it remains zero for all gains and for any combination of diode detectors connected at the input. The maximum attainable gain‐bandwidth product is somewhat lower with the JFETs (typically 400 MHz), but the amplifier operates reliably for gains up to 40–50 dB.

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