The Tektronix M377  (P/N 165-2129-03,165-2089-06,155-2089-05) is an amplifier monolithic integrated circuit designed by John Addis.

Its initial use was in the 11A16, 11A32, 11A33, 11A34 and 11A52 plug-ins.

About the M377, John Addis says:

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, and 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), 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.

(See also John's comments on the M777 successor.)

Later, the M377 and M777 were also used in several other Tek instruments. If the instrument has more than 300 MHz bandwidth at 1 mV/div sensitivity, it can be assumed to be using the M777 which makes use of Tek's later IC fabrication process that provides transistor with higher ft.

The M377 and M777 are in the following instruments based on the specified bandwidth at 1 mV/div:

2245 2245A 2247 2247A 2252
AM503A AM503B
TDS410 TDS420 TDS460
TDS520 TDS520A TDS524A
TDS540 TDS540A TDS544A


Packaging of the M377

11000 series instruments and the AM503A and AM503B use a 1.22 cm square, flat ceramic package compatible with the Hypcon connector. The ceramic rests on a heat sink supplied by the instrument’s chassis.

TDS series oscilloscopes use a J lead package with a tall heat sink. The package is soldered to the circuit board.

The ceramic-packaged M377 die has the following Tek part numbers:

Instrument Instrument SN range Part Number Output R/side Components on ceramic hybrid
11A32 B010100 - B031028 165-2089-03 100 Ω M377 chip only
11A32 B031029 - up 165-2089-05 100 Ω M377 chip only
11A33 B010101 - up 165-2089-05 100 Ω M377 chip only
11A34 B010100 - B031044 165-2089-04 200 Ω M377 chip only
11A34 B031045 - up 165-2089-06 200 Ω M377 chip only
11A52 B010100 - B010179 165-2129-02 100 Ω Diode bridge + 50 Ω input R + M377 chip
11A52 B010180 - up 165-2129-03 100 Ω Diode bridge + 50 Ω input R + M377 chip
AM503A B010101 - up 165-2456-00 100 Ω M377 chip only aka M694 (relaxed DC bal spec)
AM503B B010101 - up 165-2456-00 100 Ω M377 chip only aka M694 (relaxed DC bal spec)

Output Control Signals

The M377 has one input (differential) and three differential outputs which are identical circuits. Each output has a its own control pins for:

  • Output invert
  • Output enable. When an output is disabled, it has no offset and maintains the specified termination impedance. That is, the internal parallel output termination remains connected, but the signal current is shut off. The self-calibration of 11k systems makes use of this property.
  • High-frequency adjustment. The HF ADJ pins have 2 kΩ input resistance to ground. The nominally acceptable inputs are ±1 V to ground. Because the input resistance tracks the nichrome standard resistors inside the chip, the input is actually current sensitive, not that it makes much difference. This current affects the standing current in a feedback amplifier and hence its open loop gain. At low frequencies, the gain of the M377 is determined entirely by resistor values. At high frequencies, there is phase shift and changing the open loop gain will also change the closed loop gain, hence the transient response.

Var Gain Control Signal

The M377 has a single VAR GAIN control input, which affects all three outputs. This control signal has a range of −1 V to +1 V. −1 V results in 0 gain. +1 V results in full gain.

165-2089-xx Pinout

In the 11A32, 11A34, and 11A52, pairs of 165-2089-xx IC are placed on the board so that their outputs are facing one another, allowing their outputs to be wired in parallel with a minimum of board complexity and asymmetry. The purpose of the three output channels varies depending on the rotational orientation of the IC on the board.

  • Output 1 (pins 25 and 26) is either AUX SIG (aka AUX TRIGGER) or DISPLAY,
  • Output 2 (pins 22 and 23) of all 165-2089-xx ICs are the TRIGGER signal.
  • Output 3 (pins 19 and 20) is either AUX SIG (aka AUX TRIGGER) or DISPLAY.


Two 165-2089-xx M377 ICs facing one another in an 11A52. Output 1 of U310 is in parallel with output 3 of U410. Output 3 of U310 is in parallel with output 1 of U410.

Note that in the 11A32 and 11A34, the HF ADJUST control voltages are not set identically for the three outputs. The reason for this is that the DISPLAY and TRIGGER outputs are in parallel with the corresponding output from another M377, which forms a broadband termination. In contrast, the AUX SIG outputs are not combined, and therefore have ¼ W resistors on the PCB, shunting to ground. The ¼ W resistors become inductive at high frequencies, so the optimal HF adjustment for the AUX SIG output is different than for the DISPLAY and TRIGGER outputs.

Pin Function Notes
1 GP0 gain select bit 0
2 STB data strobe for gain and bandwidth limit
3 +15 V
4 +Vin
5 Analog ground
6 −Vin</in>
7 Analog ground
8 Var Gain
9 −5 V
10 −5 V
11 Output 1 Invert
12 Output 1 Enable
Pin Function Notes
13 Analog Ground
14 +5 V
15 Output 2 Invert
16 Output 2 Enable
17 Output 3 Invert
18 Output 3 Enable
19 −Output 3
20 +Output 3
21 Output 3 HF Adjust See "High-frequency adjustment" above
22 −Output 2
23 +Output 2
24 Output 2 HF Adjust See "High-frequency adjustment" above
Pin Function Notes
25 −Output 1
26 +Output 1
27 Output 1 HF Adjust See "High-frequency adjustment" above
28 Ground
29 Ground
30 Ground
31 +5 V
32 −5 V
33 BP1 bandpass select bit 1
34 BP0 bandpass select bit 0
35 GP2 gain select bit 2
36 GP1 gain select bit 1

Pin function legend

Power pins
Ground pins
HF signals
TTL digital control signals
analog control signals