A Flexible Gate

In researching the design of a fast comparator, I re-discovered an oft-used circuit I’d seen in Tektronix designs of the 1970s. This was a flexible gate, which essentially provided voltage level translation between elements of a circuit. For example, in waveform generation, a limit comparator provides one kind of output, but elsewhere a different range of voltages is needed. The gate provides the translation needed for interoperability.

Some sketches:

Playing with this design as a proto-comparator, I learned quite a lot about how it could be flexibly designed for application use. Some things learned:

1. The “switch point voltage” can be programmed with a resistor divider. The Zener diode used in the test circuit was for convenience, and is not strictly necessary — just acts as a specific voltage reference.
2. The emitter resistor value is sized to the desired current to be switched, after adding one Vbe to the switch point voltage, then subtracting that from Vcc, then divided by a resistor value. The ranges are quite flexible, and not too critical.
3. Within available output voltage compliance, the unloaded desired output voltage can be set directly by choosing the output resistor value.
4. On the input side, the gate can be held cutoff, or inhibited, by a clamped pull-up, as shown in the test-circuit. This clamp voltage is higher than the switch point voltage, forcing the input transistor off, and the output transistor on. As this input pull up network is only a moderate impedance, any external circuit that can yank down the input by one Vbe will switch the gate.
5. These circuits are fast, acting approximately as fast as TTL logic speeds.