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Current Feedback Using Vacuum Tubes

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Figure 1-16 below is an adaptation from a 1937 article on feedback amplifiers by Frederick E. Terman (Reference 2: Frederick E. Terman, "Feedback Amplifier Design," Electronics, January 1937, pp. 12-15, 50). Notice that the AC-coupled R2 feedback resistor for this two-stage amplifier is connected to the low impedance cathode of T1, the pentode vacuum tube input stage. Similar examples of early tube circuits using cathode feedback can be found in Reference 3 (Reference 3: Edward L. Ginzton, "DC Amplifier Design Techniques," Electronics, March 1944, pp. 98-102).
DC-coupled op amp design using vacuum tubes was difficult for numerous reasons. One reason was a lack of suitable level shifters. Multi-stage op amps either required extremely high supply voltages or suffered gain loss because of resistive level shifters. In a 1941 article, Stewart E. Miller describes how to use gas discharge tubes as level shifters in several vacuum tube amplifier circuits (Reference 4: Stewart E. Miller, "Sensitive DC Amplifier with AC Operation," Electronics, November 1941, pp. 27-31, 105-109). A circuit of particular interest is shown in Figure 1-17 (opposite).
A 1937 vacuum tube feedback circuit designed by Frederick E. Terman, using current feedback to the low impedance input cathode (adapted from Reference 2)
Figure 1-16: A 1937 vacuum tube feedback circuit designed by Frederick E. Terman, using current feedback to the low impedance input cathode (adapted from Reference 2)
In the Fig. 1-17 reproduction of Miller's circuit, the R2 feedback resistor and the R1 gain setting resistor are labeled for clarity, and it can be seen that feedback is to the low impedance cathode of the input tube. The author suggests that the closed-loop gain of the amplifier can be adjusted from 72dB-102dB, by varying the R1 gain-setting resistor from 37.4Ω to 1.04Ω.
What is really interesting about the Miller circuit is its frequency response, which is reproduced in Figure 1-18 (opposite). Notice that the closed-loop bandwidth is nearly independent of the gain setting, and the circuit certainly does not exhibit a constant gain-bandwidth product as would be expected for a traditional VFB op amp.
For a gain of 72dB, the bandwidth is about 30kHz, and for a gain of 102dB (30dB increase), the bandwidth only drops to ~15kHz. With a 72dB gain at 30kHz VFB op amp, bandwidth would be expected to drop 5 octaves to ~0.9kHz for 102dB of gain.
A 1941 vacuum tube feedback circuit using current feedback
Figure 1-17: A 1941 vacuum tube feedback circuit using current feedback
To clarify this point on bandwidth, a standard VFB op amp 6dB/octave (20dB/decade) slope has been added to Fig. 1-18 for reference.
A 1941 feedback circuit shows characteristic CFB gain-bandwidth relationship
Figure 1-18: A 1941 feedback circuit shows characteristic CFB gain-bandwidth relationship
Although there is no mention of the significance of this within the text of the actual article, it nevertheless illustrates a popular application of CFB behavior, in the design of high speed programmable gain amplifiers with relatively constant bandwidth.
When transistor circuits ultimately replaced vacuum tube circuits between the late 1950s and the mid-1960s, the current feedback architecture became popular for certain high speed op amps. Figure 1-19 below shows a fast-settling op amp designed at Bell Labs in 1965, for use as a building block in high speed A/D converters (Reference 5: J. O. Edson and H. H. Henning, "Broadband Codecs for an Experimental 224Mb/s PCM Terminal," Bell System Technical Journal, Vol. 44, No. 9, November 1965, pp. 1887-1950).
The circuit shown is a composite amplifier containing a high speed AC amplifier (shown inside the dotted outline) and a separate DC servo amplifier loop (not shown). The feedback resistor R2 is AC coupled to the low-impedance emitter of transistor Q1. The circuit design was somewhat awkward because of the lack of good high frequency PNP transistors, and it also required zener diode level shifters, and non-standard supplies.
A 1965 solid state current feedback op amp design from Bell Labs
Figure 1-19: A 1965 solid state current feedback op amp design from Bell Labs
Hybrid circuit manufacturing technology, which was well established by the 1980s, allowed the use of fast, relatively well-matched NPN and PNP transistors, to realize CFB op amps. The Analog Devices' AD9610 and AD9611 hybrids were good examples of these devices introduced in the mid-1980s.
With the development of high speed complementary bipolar IC processes in the 1980s (Reference 6: "Op Amps Combine Superb DC Precision and Fast Settling," Analog Dialogue, Vol. 22, No. 2, pp. 12-15) it became possible to realize completely DC-coupled current feedback op amps using PNP and NPN transistors such as the Analog Devices' AD846, introduced in 1988 (Fig. 1-15, again). Device matching and clever circuit design techniques give these modern IC CFB op amps excellent AC and DC performance without a requirement for separate level shifters, awkward supply voltages, or separate DC servo loops.
Various patents have been issued for these types of designs (Reference 7: David A. Nelson, "Settling Time Reduction in Wide-Band Direct-Coupled Transistor Amplifiers," US Patent 4,502,020, Filed October 26, 1983, Issued February 26, 1985 and Reference 8: Royal A. Gosser, "DC-Coupled Transimpedance Amplifier," US Patent 4,970,470, Filed October 10, 1989, Issued November 13, 1990, for example), but it should be remembered that the fundamental concepts were established decades earlier.
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