TEKTRONIX 5A22N Differential Amplifier

2. Analysis of core circuit modules

Input attenuator:

Attenuation ratio: VOLTS/DIV 0.1V-5V range with 100X attenuation, 10 μ V-50mV range with 1X attenuation;

Features: Frequency compensation, maintaining 1M Ω//47pF input characteristics, balancing common mode signals through R132 (Atten DC CMR).

Pre amplifier:

Structure: Two identical operational amplifiers form a differential circuit (Q150A/B, Q190A/B, Q200A/B);

Floating power supply: composed of Q170/Q176 (constant current source) and VR173/175/176 (Zener transistor), it maintains the stability of the amplification device operating point and improves CMRR as the common mode signal changes;

Gate current compensation: Regulate R121/R127 to offset FET gate leakage current (≤ 100pA) and avoid high-sensitivity offset (such as 100pA × 1M Ω=100 μ V offset in 10 μ V/Div mode, which may cause trajectory offset screen).

DC offset generator:

Structure: Q240/Q244/Q246A/B form a voltage comparator, with VR251 (transistor) providing a reference voltage;

Function: By adjusting COARSE (R260) and FINE (R268), offset current is generated to cancel the DC component of the input signal, with a maximum cancellation of 0.5V.

Output amplifier:

Structure: Push pull amplifiers Q348/Q352, R351 (GAIN) adjust the total gain to match the requirements of the host;

Position adjustment: Q360/Q362 (positioning current drive), R360 (POSITION) changes the current to adjust the static position of the CRT beam.

Calibration (Section 3)

1. Calibration prerequisites and preparations

Applicable scenarios: After instrument maintenance, long-term use (component aging) leads to accuracy deviation;

Environmental requirements: Temperature of 20-30 ℃, preheating for 20 minutes;

Equipment disassembly: Remove the left protective cover of 5A22N and the left panel of 5100 series oscilloscope (or use plug-in extender 067-0645-00);

Initial settings: Set the 5A22N control to POSITION median LF-3dB=1Hz、HF-3dB=1MHz、VOLTS/DIV=50mV、 Variable gear=CAL,+/- input=DC+GND, STEP ATTEN BAL median; The 5B10N time base is set to automatic triggering,+slope, and AC coupling.

2. Required testing equipment (including accessories)

Specific requirements/model examples for equipment types

Oscilloscope System 5100 Series (including 5B10N Time Base Plugin)

Constant amplitude sine wave generator frequency 2Hz-1MHz, output 0.5V-40Vp-p (such as General Radio 1310-B)

Standard amplitude calibrator 1kHz square wave, output 5mV-50V, accuracy ± 0.25% (recommended 067-0502-01)

Accessories - Coaxial Line: 50 Ω, 42 inches, BNC（012-0057-01）

-Dual input cable: matching signal path, BNC（067-0525-00）

-1000:1 voltage divider: accuracy ± 0.2% (067-0529-00)

-Input RC Normalizer: 1M Ω× 47pF (067-0541-00)

-Serial terminal: 50 Ω, accuracy ± 2% (011-0049-01)

3. Key Calibration Steps (Core 8 Steps)

Step attenuator balance:

R292 (AC STEP ATTEN BAL): Switch between VOLTS/DIV 50mV-0.1V to minimize trajectory offset;

Adjust R318 (VAR BAL): Shift the variable gear from CLOCKWISE to COUNTERCLOCKWISE to minimize trajectory deviation;

Adjust R250 (COARSE DC BAL): Set LF-3dB to DC, switch VOLTS/DIV 50mV-0.1V, and minimize trajectory offset.

Gate current regulation:

+Input to 50 Ω terminal → LF-3dB=DC → Release+GND → Switch+AC, adjust R121 (+GATE CURRENT), minimize trajectory offset;

-Input to 50 Ω terminal → press+GND → release - GND → switch - AC, adjust R127 (- GATE CURRENT), minimize trajectory deviation.

Attenuator DC common mode rejection:

Release+/- GND → VOLTS/DIV=0.1V →+/- Input through dual cables connected to a 50V square wave (calibrator) → Adjust R132 (ATT DC CMR) to display the minimum amplitude.

Input compensation:

-GND pressed ->VOLTS/DIV=50mV ->+input connected to 0.5V square wave (normalized by RC) ->C118 adjusted (Atten Time Constant), with the best square wave front;

Similarly, input C148, C145, C142.

Amplifier gain calibration:

VOLTS/DIV=10mV →+input connected to 50mV square wave → adjust R351 (GAIN), display amplitude exactly 5 grids;

Turn the variable gear to COUNTERCLOCKWISE, with a display amplitude of<2 grids, and then turn it back to CAL.

VOLTS/DIV accuracy check:

VOLTS/DIV=5V →+input connected to 20V square wave (through 1000:1 voltage divider X1 gear) → gradually decrease VOLTS/DIV, synchronously adjust the calibrator output, ensure display of 4-5 grids, accuracy ± 2%;

VOLTS/DIV=5mV gear → voltage divider set X1000 → calibrator output 20V → HF-3dB=10kHz → repeat the above checks.

Common mode rejection ratio (CMRR) calibration:

Release - GND → VOLTS/DIV=10mV →+/- input connected to 20Vp-p, 50kHz sine wave → adjust C160 (CMR 2), display minimum;

VOLTS/DIV=50 μ V → Time base=10 μ s/Div → Adjust C220 (CMR 1), display minimum;

LF-3dB=0.1kHz → switch to C210 (CMR 3), display minimum, repeat until there is no interaction effect.

Bandwidth calibration:

-GND pressed → VOLTS/DIV=1mV → LF-3dB=DC → Time base=1ms/Div →+input connected to 1kHz, 8-grid sine wave → Generator output 1MHz → C330 adjusted, display amplitude reduced to 5.6 grid (-3dB point).

Drawings and Parts List (Section 4)

1. Symbols and reference identification rules

Component symbol: Following ANSI Y32.2-1970 standard, logical symbol follows MIL-STD-806B (positive logic);