Unity Gain Emitter Follower Bridge Circuit

1. Field of the Invention

The invention generally relates to emitter follower logic circuits using complementary transistors and, more particularly, to such circuits characterized by unity gain, no level shift, low power consumption and high speed in responding to input signal transitions of either increasing or decreasing sense.

2. Description of Prior Art

As is well known, a capacitively loaded emitter follower circuit using complementary transistors such as shown, for example, in FIG. 8-36 on page 304 of the text Pulse, Digital and Switching Waveforms, by J. Millman and H. Taub, McGraw Hill, 1965, provides an output waveform having rising edges and falling edges that follow (with comparable speed) the corresponding rising and falling edges of the input waveform. In the example given, however, the bases of the series-connected complementary transistors are driven by the same input waveform whose voltage excursions turn each transistor on and off.

This necessitates a relatively large input voltage excursion, at least equal to the sum of the V.sub.be 's for forward biasing the emitter-base junctions of the two transistors, and represents a significant power expenditure.

Furthermore, the cited emitter follower circuit provides less than unity gain so that the signal losses must be overcome by auxiliary drivers in order to cascade the circuits. Additionally, the cited circuit shifts the level of the signal between input and output, by at least one V.sub.be, requiring a translator to eliminate the level shift when designing cascaded logic.

It is desirable that the complementary emitter follower circuit be modified to avoid level shift, and to provide unity gain while reducing the required input voltage excursion so as to maximize the response speed of the circuit.

A high speed, unity gain, emitter follower OR circuit introducing no level shift is achieved using two current sources, an input pair of bipolar transistors and a bridge connection of two pairs of complementary bipolar transistors. The circuit is characterized by equally fast response to rising excursions and to falling excursions of input pulsed signals. One of the current sources powers the input pair of transistors while the other current source powers the pair of complementary transistors on the input side of the bridge. By virtue of the current source and the bridge connection, the emitters of each of the transistors follow their respective bases with unity gain.

The sole FIGURE is a schematic circuit diagram of a preferred embodiment of the invention.

Referring to the FIGURE, input NPN transistors 1 and 2 are connected in parallel in conventional ORlogic fashion, with the bases being coupled to receive respective input signals applied to terminals 9 and 10. The commonly connected emitters (point A) of transistors 1 and 2 are coupled to the collector of NPN transistor 3. The emitter of transistor 3 is connected via resistor 11 to a negative potential source at terminal 12. The commonly connected collectors of transistors 1 and 2 and the base of transistor 4 are coupled to a positive potential source at terminal 13.

A first pair of complementary transistors 5 and 6 are connected emitter-to-emitter (point B). The emitters of a second pair of complementary transistors 7 and 8 are also connected to each other (point C) and to capacitively loaded (C.sub.LOAD) output terminal 14. Preferably, the V.sub.be 's and the (f.sub.t 's) of the NPN and PNP transistors have similar values so that the D.C. level shift between input terminals 9 and 10 and output terminal 14 is minimized and so that the output signal at terminal 14 follows input signals at terminals 9 and 10 with similar speed for rising as well as falling signal excursions.

The base of transistor 3 and the collectors of transistors 6 and 7 are coupled to a negative voltage source at terminal 15. The base-collector node of diode-connected transistor 5 is connected to a positive voltage source at terminal 16 via resistor 17 and the emitter-collector current path of PNP transistor 4. Transistors 3 and 4, with their associated emitter resistors 11 and 17 function as respective constant current sources for input transistor pair 1 and 2 and for the input complementary transistor pair 5 and 6 of the bridge configuration comprising transistors 5, 6, 7 and 8.

In operation, the D.C. offset of two V.sub.be 's, provided between the bases of transistors 5 and 6, match the corresponding offset required between the bases of transistors 7 and 8. Accordingly, the quiescent voltage at the emitters (point A) of input transistor pair 1 and 2 is set so that transistors 7 and 8 simultaneously are placed at their respective thresholds of conduction. Thus, assuming, for example, that input 9 is up and input 10 is down and then the signal at input terminal 9 begins to fall, causing a corresponding fall at point A, transistor 7 immediately begins to conduct while transistor 8 immediately ceases to conduct. A transient excursion of only a fraction of a V.sub.be is sufficient to produce these results whereby very fast response occurs to a falling input signal excursion as compared to state-of-the-art complementary emitter follower circuits. Similarly, when both inputs 9 and 10 are down and an input signal excursion of increasing sense occurs at terminal 9, transistor 8 immediately begins to conduct while transistor 7 immediately ceases to conduct.

It is to be noted that the current source comprising transistor 3 and resistor 11 imparts unity gain to the input emitter follower pair 1 and 2. Similarly, the current source comprising transistor 4 and resistor 17, together with diode-connected transistor 5, imparts unity gain to emitter follower 6. Thus, the potential at point A follows that of points 9 and 10 with unity gain while the potential at point B follows that of point A with the same unity gain. By virtue of the bridge arrangement of transistors 5, 6, 7 and 8 and the matched V.sub.be 's thereof, the potential of point C closely tracks that of point B also with unity gain.

The D.C. offset of one V.sub.be from the base to the emitter of NPN transistor 1 or 2 is negated by the equal and opposite D.C. offset of one V.sub.be from the emitter to the base of PNP transistor 6. Inasmuch as point C tracks point B as previously explained, there is no net D.C. offset acting between output terminal 14 and either of input terminals 9 or 10.

Typical parameter values for the circuit shown in the FIGURE are as follows: