Test Your EQ (Engineering Quotient)

EQ #19

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 input characteristics of the circuit? In other words, what sort of load does the driving circuit see?

When VIN is high, there is virtually no current flowing through the input terminal — just the leakage current through Q1’s base-collector junction.

When VIN is low, the driving circuit must sink the current set by R1 divided by the beta (current gain) of Q1. For example, if R1 is 4,300 Ω, giving a current through Q1 of about 1 mA, and the beta of Q1 is 50, then the driving circuit must sink about 20 µA.


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