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Varactor Bridge Op Amps


George Philbrick championed a novel op amp type that became a GAP/R profit maker—the varactor bridge amplifier. In this circuit, voltage variable capacitors (varactors) are used in an input stage that processes the op amp error voltage as a phase-sensitive AC carrier. By careful bridge component arrangement, the op amp input terminals are forced to see only tiny DC leakage currents, i.e., as small as 1pA (or in some cases, much less).

As a result, a varactor bridge op amp achieved the lowest input current of any op amp available in the solid-state period. Lower than common tubes, in fact! In addition, since there was no input DC path to common, the allowable input CM voltage of a varactor bridge op amp could go very high— to levels as high as ±200V.

Figure H-10 below illustrates in block diagram form a varactor bridge op amp. There are four main components, the front end composed of the bridge circuit and a high frequency oscillator, an AC amplifier to gain-up the bridge output error voltage, a synchronous phase detector to convert the amplified AC error to a corresponding DC error, and finally an output amplifier, providing additional DC gain and load drive.

The circuit worked as follows: A small DC error voltage VIN applied to the matched varactor diodes D1 and D2 causes an AC bridge imbalance, which is fed into the AC amplifier. This AC voltage will be phase sensitive, dependent upon the DC error. The remaining parts of the loop amplify and detect the DC error.

To apply the amplifier, an external feedback loop is closed from VOUT back to the inverting input terminal, just as with conventional op amps. The difference in the case of the varactor bridge op amp lies in the fact that two unusual degrees of freedom existed, in terms of both bias current and CM voltage.
Generalized block diagram for a varactor bridge solid-state op amp
Figure 10: Generalized block diagram for a varactor bridge solid-state op amp

The GAP/R varactor bridge op amp model was called the P2. It was a premium part in terms of the DC specifications, but not speed. In fact, the unity-gain frequency was just 75kHz, but the specification that people keyed on was the ±10pA input offset current. Also, the CM range of ±200V very likely enabled a few applications that previously might have required the use of a tube amplifier to address.

In 1966, an SP2A sold for a then astronomical price of $227 (see Reference 11, again). Bob Pease wrote a fascinating narrative on the P2's collaborative development at GAP/R, which was by engineers George Philbrick and Bob Malter (Reference 12: Bob Pease, "Chapter 9, The Story of the P2," within Jim Williams, Ed., Analog Circuit Design, Butterworth-Heinemann, 1991, ISBN:0-7506-9166-2). George Philbrick was also issued a patent on a varactor bridge amplifier, in 1968 (Reference 13: George A. Philbrick, "Electronic Amplifier," US Patent 3,405,366, filed June 30, 1965, issued Oct. 8, 1968 (A varactor bridge amplifier)). In 1966 GAP/R and Nexus Research Laboratories were purchased by Teledyne Corporation, and the merged product line continued into hybrids and ICs.

But, there was later on to be a sad note in this GAP/R history. GAP/R founder and master innovator George Philbrick passed away in late 1974, at a relatively young age of 61. A tribute to George Philbrick was offered by his partner and collaborator, Professor Henry Paynter of MIT (Reference 14: H. M. Paynter, "In Memoriam: George A. Philbrick," ASME Journal of Systems, Measurement and Control, June 1975, pp. 213-215).
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