App Note 21

"Composite amplifiers." 12 pages.

This application note discusses several topologies for composite amplifiers.  In a sense, it continues the theme from App Note 18 of improving the performance of op amps using additional (often discrete) gain stages.  I love these discrete designs.

Figure 1 shows the basic topology, with a precision amplifier driving the DC point of a high-speed amplifier.  This configuration is sometimes called a Goldberg amplifier, where the precision LT1012 monitors the voltage of the summing junction and drives the noninverting input of the high-speed LT1022 to correct its offset voltage.

Figure 2 uses the same approach.  A pair of discrete FETs replaces the input stage of the LT318A (using the offset pins, like the circuit in Figure 16 of App Note 13 and the low-noise preamp in National AN-299, which Jim also wrote).  Again, the LT1012 monitors the summing junction and drives the noninverting input of the FET pair to correct the offset voltage of the FETs and LT318A.  This circuit uses a lag network (10-pF and 1-k) at the summing junction for stability (and if you look closely at the step response in Figure 3, you can see the long-tail transient of the lag, impacting the settling time).

The FET probe circuit in Figure 4 is a copy of Figure 14 in App Note 9.  Figure 6 improves on the gain of Figure 4, allowing the gain to be precisely unity (or larger).  The circuit in Figure 8 is another DC-stabilized fast amplifier, using a high-speed NPN pair and a LT1010 buffer.  Figure 9 uses a PNP level-shifting stage to trade some speed for output voltage swing.  A FET buffer improves the bias current at the summing junction of the input NPN pair.

Figure 11 is a current-feedback amplifier, using LT1010 buffers as the input and output drivers, and a discrete current mirror for the gain stage.  The LT1001 provides a low-frequency offset-voltage correction loop.  The best circuit, the "Son of Godzilla Amplifier" in Figure 12, replaces the LT1010 buffers with discrete buffers.  This circuit achieves an impressive bandwidth of 110 MHz and slew rate of 3000 V/us (using 2N3904 and 2N3906 transistors!), even at a gain of 20. The two op amps provide feedback offset correction and feedback biasing ("Without closed-loop control, the circuit will quickly go into thermal runaway and destroy itself.")

The final three circuits show a few other composite designs.  Figures 14 and 16 show approaches for low-noise amplifiers, and Figure 17 shows a paralleling trick for LT1010 buffers (this trick was described by Bob Widlar in his App Note 16; see his Figure 43).

Best quote (from page AN21-3): "Note that rise time is limited by the pulse generator and not the circuit."  (He also mentions this deficiency in the caption of Figure 13.)  Jim will fix this embarrassing problem (a slow pulse generator) in a future app note!