12 August 2011

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!


Anonymous said...

Hello Jim, I would appriciate when the curves with a fast generator would finaly come.I need at least 500 MHZ with 4 Vpp at 100 Ohm. Better until 1 GHz.
I have at the moment no much times to build up a test circuit at work, But I would get time from my boss if the results from your circuit give hope to reach the target.

Best Regards from Berlin

udailey said...

Hi Kent and thanks for this tribute to Jim and the collecting of his notes. In the "son of Godzilla" notes Jim mentions to IMMEDIATELY set Iq to 80mA. Could you help me understand how/where to measure this?

udailey said...

Also, while 2N3904 and 2N3906 are still easy to come by and are very inexpensive the BC550C/BC560C by On Semi are a bit faster, have similar specs is many regards but have a much lower amount of noise. I wonder if you think there are any downsides to switching to these two transistors in the Son of Godzilla circuit.

udailey said...

Sorry to keep adding to my own commentary but I thought you might not be aware of the Internet Archive Wayback Machine.
Using this site you can search, for instance, Nat Semi's website from 2001 where App Note 272 still exists but if you tried to find it there today it obviously would not.