In some conventional signal translators, an element stack having two or more bipolar transistors serially coupled to one another between a first voltage supply and a lower-value second voltage supply is used to perform voltage translation. S. Platt et al disclose such a translator in U.S. Pat. No. 3,778,640, "Signal Voltage Level Translating Circuit". In this translator, an input voltage supplied to the base of a first NPN transistor in a primary stack is converted into an output voltage at a lower voltage level taken from the collector of a second NPN transistor in the stack. A first resistor is connected between the emitter of the first transistor and the collector of the second transistor. The collector of the first transistor is tied to the first voltage supply, while the emitter of the second transistor is tied to the second voltage supply.
A reference current source supplies a first reference voltage to the base of the second transistor to control the amount of downward voltage translation. The reference source contains an image stack consisting of a third NPN transistor, a second resistor, and a fourth NPN transistor connected to one another in the same manner as the first transistor, the first resistor, and the second transistor, respectively. A second reference voltage is supplied to the base of the third transistor. The collector of the fourth transistor is tied back to its base from which the first reference voltage is provided. The resistors have substantially the same resistance.
During operation, all of the transistors are conductive. When the input voltage equals the second reference voltage, the output voltage is at a selected nominal value below the input voltage. Because the bases of the second and fourth transistors are connected together and their emitters are tied directly to the second voltage supply, the current through the second transistor equals the current through the fourth transistor. Accordingly, a change in the input voltage above or below the second reference voltage results in the same change in the output voltage.
This translator provides linear voltage translation. However, the first reference voltage does not vary with the first supply voltage since it does not affect the base-to-emitter voltage of the fourth transistor. As a consequence, the output voltage changes as the first supply voltage varies. This is undesirable in some applications.