This video provides a detailed circuit analysis of a vintage Tektronix Type 1101 accessory power supply, likely designed in the 1960s. The presenter meticulously explains the voltage regulation for the +15V output, focusing on classic discrete transistor design before the advent of integrated op-amps and three-terminal regulators. A key highlight is the in-depth demystification of the unique implementation of an NPN pass transistor within the ground return path of a positive supply. The video educates on the underlying principles of negative voltage biasing and "upside-down" emitter follower configurations necessary to understand this ingenious design choice, offering valuable lessons in fundamental analog electronics.
Tektronix Type 1101 Power Supply: Dissecting Vintage Transistor Regulation
0:00 Introduction to Tektronix Type 1101: The presenter introduces a Tektronix Type 1101 accessory power supply, a vintage unit likely designed in the 1960s and produced in the 1970s. It is presented as an excellent educational tool for understanding discrete transistor design, as it predates modern IC-based components like op-amps and three-terminal regulators.
1:34 Power Supply Architecture: The unit features a transformer with separate windings for +15V, +5V, and -5V outputs. Each output utilizes a bridge diode rectifier, followed by capacitors to smooth the raw DC voltage, and then a dedicated discrete regulator circuit.
1:54 Core Regulator Components: The central part of the regulator circuit is a "comparator" built using a differential pair of matched transistors, which functions similarly to an operational amplifier. This comparator feeds an error amplifier stage, ultimately controlling a pass transistor.
2:48 Detailed +15V Supply Analysis: Focusing on the +15V output, the circuit uses a bridge rectifier and two high-quality, expensive 160Β΅F solid tantalum capacitors in parallel to generate the raw DC. The regulator aims to precisely crop this voltage down to 15V. This supply provides up to 400mA.
4:41 Voltage Reference and Feedback Loop: A 9V Zener diode establishes a stable reference voltage for the differential pair comparator (measured at 8.8V on the schematic). The 15V output is fed back through a resistor divider (with a trim pot for adjustment) to match this 8.8V reference, ensuring the comparator is balanced when the output is at the desired 15V.
6:41 Error Voltage Amplification: The small "error voltage" swing from the differential pair (from 6V at no load to 0.5V at max load) is then amplified by a subsequent transistor stage. This amplifier, using a 0.7V diode reference, converts the small input swing into a much larger output swing (-3.6V to -8.2V).
8:56 The Unusual Pass Transistor Placement: The amplified error voltage controls a pass transistor responsible for dropping the excess voltage. Crucially, this NPN pass transistor is placed on the minus (ground return) side of the positive 15V supply, which seems counter-intuitive for passing positive current.
11:20 Essential Background: Emitter Followers & Biased Batteries: To explain the pass transistor's operation, the video introduces two key concepts:
Emitter Followers: Standard NPN emitter follower and an "upside-down" version with its collector at ground and operating with negative input/output voltages.
Battery Biasing: How a floating 30V battery's voltage relative to ground changes if one of its terminals is tied to a non-zero voltage (e.g., tying the negative terminal to -15V results in a +15V output relative to ground from the positive terminal).
17:18 Resolving the Pass Transistor Mystery: The raw DC stage generates 30V across its capacitors, acting as a "floating" 30V battery. This "battery" is effectively biased by the -15V supply, setting its bottom terminal at -15V, which in turn makes its top terminal +15V relative to ground. The NPN pass transistor is then positioned in the negative current return path of this biased 30V supply. It receives negative voltages at its base (-14V to -8.7V) from the preceding stages, allowing it to correctly regulate the current in this "backwards" configuration. This design choice likely enables the differential amplifier to operate around 0V.
20:21 Further Exploration: The presenter encourages interested viewers to download the Type 1101 service manual from the Tek Wiki for advanced learning. He briefly mentions additional circuit elements not covered in detail, such as current limiting, shutdown, and temperature compensation features for each supply.