This video provides a detailed circuit analysis of a vintage Tektronics Type 1101 accessory power supply, emphasizing the educational value of its 1960s-era discrete transistor design. The presenter focuses on the +15V linear regulator, explaining its core components, including a differential pair acting as an error amplifier, a Zener diode reference, and a pass transistor. The central focus is on the power supply's unconventional and counter-intuitive design, which places an NPN pass transistor on the low side (ground return) of the positive output. Through a series of analogies involving emitter followers and referenced batteries, the analysis reveals that the circuit generates a floating 30V potential and regulates the output by controlling the negative voltage on the low side of this potential, thereby forcing the high side to a stable +15V relative to ground.
Analyzing the Tektronics 1101: A Lesson in Vintage Power Supply Design
(0:00:07) Learning from Vintage Electronics: The video introduces a Tektronics Type 1101 power supply, arguing that its pre-IC, discrete transistor design is an excellent tool for learning fundamental electronics principles that are obscured in modern IC-based circuits.
(0:01:34) Regulator Circuit Overview: The supply features separate transformer windings and regulator circuits for its +15V, +5V, and -15V outputs. The design uses a differential pair of matched transistors as an error amplifier, functioning like a discrete op-amp.
(0:04:41) Voltage Regulation Explained: For the +15V supply, the circuit compares a stable 9V reference from a Zener diode against a divided-down sample of the output voltage. The error amplifier adjusts the circuit to keep these two voltages equal, thereby maintaining a constant +15V output.
(0:07:18) Error Amplification: The small voltage change from the differential pair is fed into a second transistor stage that provides significant gain, creating a larger control signal for the main pass transistor.
(0:09:11) A Counter-Intuitive Design: The primary puzzle of the circuit is its use of an NPN pass transistor on the low side (ground return path) of a positive power supply, a configuration that at first glance appears unable to conduct current correctly.
(0:14:05) The "Floating Battery" Concept: The presenter explains that the transformer and rectifier create a floating 30V potential across the main filter capacitor, much like a 30V battery. The output voltage is not taken directly from this but is instead referenced to ground by the rest of the circuit.
(0:18:04) The Circuit's Clever Solution: The regulator works by controlling the voltage on the negative side of this floating 30V supply, biasing it to approximately -14V. Because there is a 30V difference, this forces the positive side to be at +15V relative to ground.
(0:19:25) The Pass Transistor's True Role: The NPN pass transistor on the low side correctly functions as an "upside-down" emitter follower, regulating the flow of negative current from ground to this negative rail, which in turn stabilizes the final +15V output.
(0:20:45) Additional Features: The complete circuit also includes a temperature-stabilized current limiting and shutdown circuit to protect the power supply from overload.