Test Your EQ (Engineering Quotient)

EQ #18

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 are the functions of resistors R1 and R2?

R1, combined with VTH, sets the amount of current flowing through Q2 to drive the base of Q3. This current should be sufficient to drive Q3 into saturation, given the expected load on it (R3 plus the external circuit).

R2 serves to make sure that Q3 isn’t turned on by any leakage current through Q2 when it’s supposed to be off. For example, a value of 100K ohms would bypass currents of up to 6 µA or so.


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