GB2084417A

GB2084417A - A Trigger Circuit

Info

Publication number: GB2084417A
Application number: GB8030398A
Authority: GB
Original assignee: Post Office
Current assignee: Post Office
Priority date: 1980-09-19
Filing date: 1980-09-19
Publication date: 1982-04-07
Also published as: GB2084417B

Abstract

A trigger circuit is described having two long tailed pair circuits (1, 2) each comprising a pair of transistors the emitters of which are connected jointly to a respective common current source for the pair. An input terminal (17) is connected to the base of the first transistor (11, 22) of both pairs. D.C. connections (31, 32; 35, 36) are provided from the first transistor (11, 22) of each pair to the base of the second transistor (21, 12) of the other pair. The voltage swing on change of state at the collector of the first transistor of one pair is smaller than that at the collector of the first transistor of the other pair and sets the hysteresis for changes of state of the other pair in response to changes in voltage applied to the input terminal. <IMAGE>

Description

SPECIFICATION A Trigger Circuit This invention relates to trigger circuits.

In many areas of digital transmission the need arises for threshold detector and one form of threshold detector which is commonly used is a Schmitt trigger circuit. A disadvantage of a conventional Schmitt trigger circuit for high switching speeds is that to avoid operating the transistors in their saturation regions the hysteresis loop width is large, typically 600 mV, so that the circuit requires a correspondingly large input signal amplitude and the timing of the switching operation is correspondingly unreliable.

Attempts have been made to obtain a narrow threshold window so as to improve the reliability of the switching of the circuit by biassing the circuit within the hysteresis region, but it has been found that this can make the circuit unstable.

It is an object of the present invention to provide an improved trigger circuit.

According to one aspect of the present invention there is provided a trigger circuit including first and second long tailed pair circuits, both connected to respond to an input signal, in which the voltage swing at the output of the second circuit when it changes state is smaller than that at the output of the first circuit and is fed to the first circuit to determine the hysteresis of the first circuit, and a connection is provided causing the second circuit to change state when the first circuit changes state.

According to a second aspect of the present invention there is provided a trigger circuit including first and second long tailed pair circuits, each having first and second transistors with their emitters connected together and to a current source for the pair, an input terminal connected to the base of the first transistor of both pairs, a first d.c. connection from the collector of the first transistor of the first pair to the base of the second transistor of the second pair, and a second d.c. connection from the collector of the first transistor of the second pair to the base of the second transistor of the first pair, wherein the voltage swing on change of state at the collector of the first transistor of the second pair is smaller than that at the collector of the first transistor of the first pair and sets the hysteresis for changes of state of the first pair in response to changes in a voltage applied to the input terminal.

The d.c, connections may take the form of emitter follower circuits and may include one or more diodes in the emitter lead of the emitter follower to provide a voltage offset equal to the forward conduction potentials of the diodes. An output from the circuit may typically be derived from the collector of the second transistor of the first pair.

The current sources for the long tailed pair circuits may be provided by large valued series resistors or suitably controlled series connected transistors.

The circuit may be constructed as an integrated circuit, in which case any diode included in the circuit would be formed by a diode connected transistor.

In order that the invention may be fully understood and carried into effect it will now be described with reference to the singe figure of the accompanying drawing which shows in diagrammatic form the circuit of one example of the present invention.

Referring to Figure 1, the circuit includes two long tailed pair circuits 1 and 2. In circuit 1 there are provided two transistors 11 and 12 having their emitters connected together and to a current source 1 3. The collectors of the transistors 11 and 1 2 are connected to a supply conductor 14 through equal value resistors 1 5 and 16 respectively. The base of the transistor 11 is connected to an input terminal 1 7. The current source 1 3 is connected to a supply conductor 18.

In the circuit 2 there are provided two transistors 21 and 22 having their emitters connected together and to a current source 23 also connected to the conductor 18. The collectors of the transistors 21 and 22 are connected through resistors 24 and 25 to the conductor 14, the resistors 24 and 25 having the same value as the resistors 1 5 and 1 6. The base of the transistor 22 is connected to the input terminal 1 7.

A d.c. connection is provided from the collector of the transistor 11 to the base of the transistor 21 including an emitter follower circuit formed by a transistor 31 having its collector connected directly to the conductor 14 and its emitter connected through a diode 32 to the base of the transistor 21. The base of the transistor 21 is connected to the supply conductor 18 through a resistor 33. A second d.c. connection is provided from the collector of the transistor 22 to the base of the transistor 12 and includes a second emitter follower formed by the transistor 35 having its collector connected directly to the conductor 14 and its emitter connected through a diode 36 to the base of the transistor 12. The base of the transistor 1 2 is connected to the conductor 18 through a resistor 37 of equal value to the resistor 33.

An output terminal 40 is connected to the collector of the transistor 12.

An input voltage V is applied to the terminal 17 and output voltage To is derived from the terminal 40. Potentials V, and V, are maintained on the conductors 18 and 14 respectively. The voltages applied to the bases of the transistors 21 and 1 2 are V1 and V2 respectively. The current source 1 3 provides a current 1, and the current source 23 provides a current 12.

In the operation of the circuit of Figure 1 , the circuit has two stable states in which the corresponding switching conditions are defined by the following relationships.

If V,-VS 100 mV And if (Vs1 .2)-V1 > 1 00 mV Then V1=V2=V9-1 .2 The 100 mV threshold value in the first two expressions corresponds to the voltage swing needed at the base of a transistor to switch it from "off" to "on". The 1.2 volts which appears in the second and third expressions represents the difference in voltage between, for example, the collector of the transistor 22 and the base of the transistor 12 due to the forward conduction potentials of the base emitter diode of the transistor 35 and the diode 36.

For the long tailed pair 1 to change state the voltage Vj must exceed V9-1 .2 volts, since V2=Vs1 .2 V. When the pairs have switched the voltages V1 and V0 assume the values V1=V,-(l1R+1 .2) V2=Vs(12R+1.2) It follows that the reverse change of state will occur when ViV1-(l2R+1 1.2) It follows from a consideration of the above expressions that the width of the hysteresis loop of the circuit is equal to l2R. In other words, the voltage swing at the collector of the transistor 22 occurring on change of state defines the width of the hysteresis loop of the long tailed pair circuit 1.

It is necessary for 12R to be less than l,R for this to occur, otherwise the pair 2 will switch before the pair 1 and the roles of the two pair circuits will be exchanged. The second d.c. coupling between the pair circuit is provided to ensure that both circuits switch at the same time. It will be apparent that the output voltage swing on change of state will be equal to l,R. The magnitudes of the currents 1, and 12 and the values of the resistors 15, 16, 24 and 25 may be modified as required to provide different circuit performances.Typically, the resistors 1 5, 16, 24 and 25 are 50Q and the resistors 33 and 37 330Q. The current from the source 1, is 30 mA and that from the source 12 has been adjusted over a range from 3 to 30 mA.

With the circuit described an output signal rise time of 200 ps has been obtained.

The circuit described is particularly suited to construction in integrated circuit form provided that the diodes 32 and 36 are formed by diode connected transistors. The current sources 13 and 23 may be included in the integrated circuits and, if desired, external connections may be provided to permit adjustment of the currents.

Although the invention has been described with reference to a specific embodiment, it is understood that modifications may be made to it without departing from the invention.

Claims

1. A trigger circuit including first and second long tailed pair circuits, both connected to respond to an input signal, in which the voltage swing at the output of the second circuit when it changes state is smaller than that at the output of the first circuit and is fed to the first circuit to determine the hysteresis of the first circuit, and a connection is provided causing the second circuit to change state when the first circuit changes state.

2. A trigger circuit including first and second long tailed pair circuits, each having first and second transistors with their emitters connected together and to a current source for the pair, an input terminal connected to the base of the first transistor of both pairs, a first d.c. connection from the collector of the first transistor of the first pair to the base of the second transistor of the second pair, and a second d.c. connection from the collector of the first transistor of the second pair to the base of the second transistor of the first pair, wherein the voltage swing on change of state at the collector of the first transistor of the second pair is smaller than that at the collector of the first transistor of the first pair and sets hysteresis for changes of state of the first pair in response to changes in a voltage applied to the input terminal.

3. A circuit according to claim 2 in which the d.c. connections include emitter follower circuits.

4. A circuit according to claim 3 wherein at least one diode is included in the emitter connection of each emitter follower circuit so as to insert a voltage offset equal to the forward conduction potential of the diode.

5. A circuit according to any of claims 2 to 4 having an output terminal connected to the second transistor of the first pair circuit.

6. A circuit according to any of claims 2 to 5 wherein the current fed from the particular current source to the second pair circuit is smaller than that fed from the current source of the first pair circuit to that circuit.

7. A circuit according to any of claims 2 to 6 wherein the current sources are formed by large value series resistors connected from a supply conductor.

8. A circuit according to any of claims 2 to 6 wherein each current source is formed by a suitably controlled transistor having its emitter collector paths connected from a supply conductor to the emitters of the transistors formed by the pair circuit.

9. A circuit according toclaim 8 wherein the current from the current source of the second pair circuit is adjustable.

10. A trigger circuit substantially as described herein with reference to the single figure of the accompanying drawing.

11. A trigger circuit according to any preceding claim constructed as an integrated circuit in which any diode is formed by a diode connected transistor.