3A9

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The Tektronix 3A9 is a differential amplifier plug-in introduced in 1969 for 560-series scopes. It shares similarities with the 3A10 transducer amplifier and the 7A22 from the 7000 series and 5A22N from the 5000-series scopes.

The 3A9 features a variable high-frequency cut-off from 100 Hz to 1 MHz and a variable low-frequency cut-off from DC to 10 kHz. When DC coupled, the 3A9 provides a variable offset voltage. The common mode rejection is 100,000:1.

Key Specifications

Bandwidth	1 MHz, LF limit switchable DC, 0.1 Hz to 10 kHz in ×10 steps, HF limit switchable 100 Hz to 1 MHz in ×3/×10 steps
Deflection	10 μV/Div to 10 V/Div in 1–2–5 sequence
Input impedance	1 MΩ // 47 pF (either input)
Signal ranges

Range	Differential signal	DC Offset	Common mode
10 μV/Div to 10 mV/Div	±1 V	±1 V	±10 V
20 mV/Div to 0.1 V/Div	±10 V	±10 V	±100 V
0.2 V/Div to 1 V/Div	±100 V	±100 V	±500 V
2 V/Div to 10 V/Div	±1000 V	±1000 V	±500 V

Internals

The differential amplifier design is a variant of the 1A7A circuit, also used with modifications in the 3A10, 7A22, 5A22N oscilloscope plug ins, and 5030/5031 scopes. The AM502 and the 26A2 for the 2601 mainframe use BJTs for the offset current sources instead of JFETs. The 26A2 adds a common mode signal circuit which is connected to a LEMO probe connector.

The 3A9 provides differential high-impedance voltage inputs and an AC current probe input for use with 125-turn probes (P6021 or P6019).

The front-end of the 3A9 starts with a differential pair formed by the two N-channel JFETs in a 151-1027-00, Q133.

Pictures

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  Preamp (front-end) schematic