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The Tektronix M55  (P/N 155-0032-00,155-0032-01) is a variable-gain transconductance amplifier monolithic integrated circuit in a 16-pin DIP, designed by Roy Hayes. The 155-0216-00 and 155-0217-00 (M55D aka M055D) appear to be improved successor versions.

Regarding the 155-0032-xx, John Addis says,

The 155-0032-00’s IC process had an FT of 1.0 or 1.2 GHz (don’t remember), rather inadequate for the job. The on-chip resistors were diffused and thus had too poor a TC and tolerance for use as gain-setting resistors.

As you can see, there are two separate gain selections determined by which set of emitters is chosen, pins 1 and 16 or 2 and 15. This selection is made off chip by choosing Q205 or Q215 to carry the current through R212. Pot R217 allows dc balance at 5 mV/div to match that at all other sensitivities, controlled elsewhere.

Pin 12 ultimately selects which pair of the four output cross-connected transistors passes the signal current from the two transistors whose bases are connected to pin 4. Which pair of output transistors is chosen determines the polarity of the output signal. This same pair of transistors also enables variable gain by allowing a controlled amount of cross-coupling of the signal.

This is an undesirable way of doing variable gain control, but neither Roy nor I knew that at the time. It is a noisy configuration. At the 0% signal gain there is lots of noise gain. It has poor thermal balance at anything other than 100% and the not very useful 0% signal gain. In between 0% and 100% gain, the circuit is poor in noise and thermal balance.

I went on to use the same (Gilbert Multiplier) circuit in the 155-0078-xx (M84) and made the same bad choice again in the 7A29. In our defense, with just two stages and the need to do both invert and variable gain control somewhere, there was not a lot of choice.

It was not until I designed the M377 for the 11000 series of plugins (11A32, 11A33, 11A34, 11A52) that I finally got it right. (See the British magazine Electronic Engineering, September, 1988, pp 40-41.)

The M377 was a single channel plugin on a chip with 0 V common mode input and output voltages, excellent variable gain control, two four pole bandwidth limits, three outputs which could be separately inverted and turned on or off.

It was the first wideband analog IC with level shift on chip (allowing 0 V common mode input and output and without PNP transistors), first wideband amplifier with any on-chip bandwidth limit selection, first to have more than two fixed gain settings (it has six), and first to require only one transient response adjustment (no thermals), and the first to have a highly linear relationship between a control voltage and gain. It also had excellent overdrive recovery. Live and learn.

The input differential amplifier's emitters are brought out to pins 1 and 16. Off-chip components provide frequency compensation and determine the gain. The chip has an input to select normal or inverted phase and an input to vary the "position", i.e., output current offset.

  • Pin 1: emitter current source
  • Pin 2: gain switching
  • Pin 3: input balance
  • Pin 4: bias
  • Pin 5: −8 V
  • Pin 6: position current source
  • Pin 7: ground
  • Pin 8: signal out −
  • Pin 9: signal out +
  • Pin 10: position
  • Pin 11: +8 V
  • Pin 12: norm / -inv
  • Pin 13: +5 V
  • Pin 14: signal in −
  • Pin 15: gain switching
  • Pin 16: emitter current source

Used in

  • The 155-0032-00 is used in the front end of the 335 signal path, just after a JFET high-impedance buffer. It is discussed on page 3-5 of the 335 manual.
  • The 155-0032-00 is also used as the trigger amplifier in the 465. In this application, the input differential amplifier has a total tail current of about 11 mA.
  • The 155-0032-00 is used as a vertical gain stage in the 7A12.
  • 475 (U520, U720)
  • FG504 (U770)
  • 1440 (-01), 1460 (-01), 1478 (-01)
  • 1480, 1481, 1482, 1485
  • 464, 465, 465B, 466, 475, 475A, 475M
  • 634, 650, 651, 652, 653, 655, 656, 670, 671
  • See also 155-0217-00