Many of my preamplifiers have been driven by high-impedance (i.e. over 1 kilohm) sources, such as broadband stub antennas, capacitive proximity sensors, Faraday cups, photodiodes, and APD’s (avalanche photodiodes). For this type of source, I have always wished for an FET-input op amp that had low equivalent input voltage noise like the best bipolar-input op amps.
A bipolar-input op amp could load down the source, reducing the voltage it presents to the op amp input, and also would inject too much noise current (typically on the order of a picoamp per root Hz) back to the source, producing an added equivalent noise voltage, which would be proportional to the source impedance. FET op amps have a very high input resistance, which avoids loading the source, and also they only inject femtoamps of noise current per root Hz back into the source.
An FET-input op amp is the best choice for high-impedance sources. However the commercially available ones have equivalent input noise voltages of about 5 nanovolts per root Hz.
Now, in http://www.linear.com/product/LT1028, Linear Technology shows us how create an effective FET-input op amp from a discrete junction FET and a bipolar op amp.
The circuit is
Synthetic FET input op amp
The data sheet for the discrete N-channel junction FET BF862 is http://www.nxp.com/documents/data_sheet/BF862.pdf
C1 represents the source impedance. the circuit will work if the source impedance is partially (a resistor and capacitor in series or parallel) or totally resistive, also.
The JFET is used as a source follower to lower the effective source resistance seen by the bipolar op amp LT1028. The total equivalent input noise voltage for this synthesized FET-input op amp is the quadrature sum of the two noise voltage sources, the JFET and the op amp: sqrt (0.8e-9^2 + 0.85e-9^2) = 1.17 nanovolt/root Hz.
The discrete FET does add phase lag to the forward transfer function of the synthesized amplifier. However, the FET has a transition frequency of 715 MHz., so phase lag at 50 MHz. will probably be under 0.1 radians (6 degrees). The gain-bandwidth product of the LT1028 op amp is 50 MHz., so the closed loop gain even in a unity gain configuration will be under one, for frequencies above 50 MHz., and thus excess phase lag above 50 Mhz will not cause instability.
The BF862 is a particularly good JFET for this purpose, having low voltage noise and high bandwidth. Other op amps could be used instead of the LT1028, where needed, but if they have a gain-bandwidth product above 50 MHz the synthesized amplifier may not be stable.
As always, this amplifier should be modeled in Spice (I suggest using LTSpice IV http://www.linear.com/designtools/software/#Amp), using worst-case parameters.
Larry MIller
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