Test Your EQ (Engineering Quotient)

EQ #20

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.

The circuit in Figure 2 was proposed as an alternate. In what ways is it better? In what ways is it worse?

Figure 2

Better: The component count is reduced by one transistor.

Better: VTH is stabilized by the forward drop of a diode, making it less dependent on the exact value of the positive supply.

Worse: The switching voltage is now determined by the combination of D1 and the base-emitter drop of Q2, both of which vary with temperature.

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Worse: Now the driving circuit must supply all of the base current for Q3 when it is in the high state.


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