Test Your EQ (Engineering Quotient)

EQ #17

Figure 1

A need came up recently in conjunction with the restoration of an antique electronic computer that uses some odd logic levels for a circuit that would translate from 0 V/+5 V logic to 0 V/–5 V logic. In other words, when the input is at 0 V, the output should be at 0 V, and when the input is at +5 V, the output should be at –5 V. The circuit in Figure 1 was proposed. Let’s dig into its operation in detail.

What is the function of the Q1-Q2 transistor pair, and what is the significance of the node labeled “VTH”?

Q1 and Q2 are wired as a differential amplifier, but in this digital application, it’s probably better to think of them as a current switch. Whenever VIN < VTH, the current through R1 is shunted to ground, and whenever VIN > VTH, the current passes instead through Q2 to the base of Q3, turning it on.

As long as Q1 and Q2 are reasonably well matched, VTH is the input voltage at which the switchover occurs, and this is relatively stable with respect to temperature. For example, if VIN is being driven by standard TTL logic, you might set VTH to 1.5 V.


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