GB2036487A - Switching Circuit - Google Patents

Abstract

At least one further semiconductor junction (24) is connected in series with the emitter of a switching transistor (17) to increase the switching voltage capability. Inverse collector current is prevented by a common base control transistor (21). When 12V is applied to terminal 20, collector current saturate from 21 saturates 17, 24 effectively earthing terminal 15 to intercept the bias to tapehead 11. When the 12V is removed, collector circuit from 21 ceases, turning 17, 24 off to generate an effective open circuit for voltages of either polarity at 15. When 15 is negative the collector/base of 24 is reverse biassed, and when 15 is positive the collector base of 17 is reverse biassed, but in neither case are the base emitter junctions forward biassed, so preventing current flow. A further embodiment substitutes a diode for transistor 24. <IMAGE>

Description

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SPECIFICATION Improvements in Switching Circuits The present invention relates to switching circuits and especially to transistor switching circuits.

It is known that in certain circumstances, a junction transistor can be used as a switch. Fig. 1 of the accompanying drawings shows at (a) an NPN junction transistor, and at (b) a PNP junction transistor. The conventional symbols are used so that the respective base, emitter and collector of the transistors can be identified. It will be seen that in each case, a resistor is included in the lead to the base of the transistor as is usual to determine the value of base current when the transistor is conducting. To render the NPN transistor conducting, the base electrode thereof must be made positive relative to the emitter electrode thereof. If the base electrode is positive relative to both the collector electrode and the emitter electrode, the NPN transistor operates in a saturated mode and its collector electrode is at substantially the same voltage as its emitter electrode.Similarly, the base electrode of the PNP transistor must be made negative relative to its emitter electrode, and if the base electrode is negative relative to both the collector and emitter electrodes, the PNP transistor conducts in its saturated mode. Hence it will be seen that both types of a transistor have a mode of operation equivalent to a closed switch between their collector and emitter electrodes.

To reduce conduction between the collector and emitter electrodes of the transistors to the lowest possible level, it is necessary to make the base electrode of the NPN transistor negative relative to both the emitter and the collector electrodes and to make the base electrode of the PNP transistor positive relative to both its collector and emitter electrodes. However, in practice, there is a limit to the range of voltages at the collector and emitter electrodes for which the base electrode can be made either sufficiently negative or sufficiently positive without damaging the transistor. Silicon transistors are typically manufactured with a collector to base breakdown voltage of 20 or more volts and an emitter to base break-down voltage of only about 5 or 6 volts.This fact and the fact that junction transistors are capable of conducting in the reverse direction between their nominal collector at nominal emitter results in it being impossible to use the transistor as a switch in situations in which the collector electrode may be subjected to voltages which would require the emitter-base junction of the transistor to be subjected to a reverse bias or more than the 5 or 6 volts of the emitter-base break-down voltage to ensure that the collector base junction would not become forward biased.It will be seen that the circumstances in which the NPN transistor cannot operate as a switch in the off state are those in which a negative voltage of more than about 5 or 6 volts relative to the emitter voltage is applied to the collector electrode, and in the case of the PNP transistor when a positive voltage of more than about 5 or 6 volts relative to the emitter voltage is applied to the collector electrode. The transistor must be allowed to conduct in its inverse active region of operation in these circumstances. The collector to emitter break-down voltage when a transistor is conducting in the reverse direction to its normal direction of conduction between the collector and emitter is usually only slightly less than the collector to base break-down voltage for the particular transistor.Consequently, it is possible to allow the transistor to conduct in this manner with a forward bias on the collector-base junction of up to 15 volts or more.

According to the present invention, there is provided a switching circuit including a switching terminal connected to the collector of a first junction transistor, a control switch having one terminal thereof connected to the emitter of a second junction transistor of the opposite type to the first junction transistor, the collector of the second transistor being resistance coupled to the base of the first transistor, the base of the second transistor being resistance coupled to a common terminal, and the emitter of the first transistor being coupled to the common terminal through a semiconductor device including a junction poled in the same direction as the emitter-base junction of the first transistor.

Preferably the said semiconductor device is a third junction transistor which is of the same type as the first junction transistor, which has its base resistance-coupled to the collector of the second transistor, and preferably has its emitter connected to the emitter of the first transistor.

An important application of a preferred embodiment of the invention is in switching an input to a record/playback head of a magnetic tape recorder for the record and playback modes of operation of the recorder. A useful feature of the preferred embodiment in this application is the low offset voltage which appears when the switching circuit is operating as a closed switch and the tape recorder is operating in its playback mode.

The invention will now be described in more detail, solely by way of example, with reference to Figs. 2, 3 and 4 of the accompanying drawings, in which~ Fig. 2 is a circuit diagram of part of the circuit of a magnetic tape recorder including a switching circuit embodying the present invention, Fig. 3 is a circuit diagram of an embodiment of the invention which presents a relatively high offset voltage in its closed state, and Fig. 4 is a circuit diagram of part of an embodiment of the invention, showing a different form of control switch from a form used in Figs. 2 and 3.

Fig. 2 shows an example of the invention embodied in a switching circuit for a magnetic head 11 serving as both record and playback head in a tape recorder, not shown. The magnetic head 1 1 has an input terminal 12 for audio 5 10 15 20 25 30 35 40 45 50 55 60 65 frequency and bias frequency signals, and an output terminal 13 for signals produced by the head 11 in response to scanning of a magnetic tape (not shown) bearing recorded signals. An audio and bias input terminal 14 is connected to the input terminal 12 at a switching terminal 15 by an input lead 16. The switching terminal 15 is connected to the collector of a first, NPN transistor 17.A single pole, two-way control switch 18 having its movable contact 19 connected to a positive twelve volt supply (not shown) has one fixed contact 20 thereof connected to the emitter of a second, PNP transistor 21. The collector of the PNP transistor 21 is coupled by a resistor 22 to the base of the NPN transistor 17, and the base of the transistor 21 is coupled by a resistor 23 to ground. The emitter of the first transistor 17 is coupled to ground through a third, NPN transistor 24 having its emitter connected directly to the emitter of the first transistor 17, its collector connected directly to ground, and its base coupled by a resistor 25 to the collector of the second transistor 21.

The output terminal 13 of the head 11 is connected to an input terminal of an amplifier 26 the output of which may be coupled to a loud speaker (not shown). Also connected to the output terminal 13 is the collector of another NPN transistor 27 which has its emitter connected to ground and its base coupled by a resistor 28 to a terminal 29 of the control switch 18. A resistor 30 couples the contact 29 to ground. Another resistor 31 couples the contact 20 of the switch 18 to ground.

The movable contact 19 of the switch 18 is shown in Fig.2 in the position in which it is in contact with the terminal 29 so that the positive 12 volts supplied is coupled through the resistor 28 to the base of the transistor 27, thereby forward biasing its emitter-base junction. The output terminal 13 of the head 11 is thus connected to ground and no output reaches the amplifier 26. In this same position of the moveable contact 19 of the switch 18, the contact 20 of the switch 18 is at ground potential through its coupling by the resistor 31 and so therefore also is the emitter of the PNP transistor 21.The emitter-base junction of the transistor 31 is consequently not forward biased, both the base and the emitter being at ground potential. Audio frequency and bias frequency signals supplied to the terminal 14 appear at the collector of the NPN transistor 17. When these signals are at positive voltages relative to the ground, the collector-base junction of the transistor 17 is reversed biased but the emitter-base junction of that transistor is not forward biased, and consequently the transistor 17 is able to act as a switch in the off condition.

The voltages at the emitters of the transistors 17 and 14 and the collector of the transistor 21 will then be at substantially ground voltage. When the audio frequency and bias frequency signals at the collector of the transistor 17 are negative relative to ground, the collector-base junction of the transistor 17 will be forward biased and these negative voltages will appear at the base and emitter of the transistor 17, the base and emitter of the transistor 24, and the collector of the transistor 21. However, in these circumstances, the collector-base junction of the transistor 24 is reversed biased and its emitter-base junction is not forward biased, and the collector base junction of the transistor 21 is reversed biased and its emitter-base junction is not forward biased.Consequently, the transistor 17 is operating in its inverse mode of operation and the transistors 21 and 24 are operating as switches in the off state. None of the emitter-base junctions are subjected to large reverse biasing, and large negative voltages which may appear at the collector of the transistor 17 are blocked by the collector-base junctions of the transistors 21 and 24. These large negative voltages may be 20 volts or more, depending upon the collector-base break-down voltages of the transistors 21 and 24.

When the moveable contact 19 of the switch 18 is moved to the position in which it is in contact with the fixed contact 20 of the switch .19, the emitter-base junction of the transistor 27 is no longer forward biased and the transistor 27 presents sufficient impedance to allow signals appearing at the output terminal 13 of the head 11 to reach the amplifier 26.

The emitter of the transistor 21 is directly connected by the switch 18 to the positive 12 volt supply and the emitter-base junction of this transistor 21 is thus forward biased so that the transistor 21 is in its iow impedance condition and the emitter-base junction of the transistor 17 and the collector-base junction of the transistor 24 become forward biased. The impedance between the switching terminal 15 and ground presented by the transistors 17 and 24 is now low and the input terminal 12 is effectively held at substantially ground potential, current flowing into or out of the collector electrode of the first transistor 17 to the switching terminal 15 according to whether the signals supplied to the terminal 14 are tending to rise positively or negatively relative to ground. The connection of the emitter electrode of the third transistor 24 to the emitter electrode of the first transistor 17 ensures that the small impedance presented between the switching terminal 15 and ground by these two transistors in their low impedance condition does not vary with the polarity of the signals applied to the terminal 14.

In a constructed example of a circuit as shown in Fig.2, the various components of the circuit were as follows~ Transistors Resistors 17 BC 237 NPN 22 33 kilohms 24 BC 237 NPN 23 5.6 kilohms 21 BC 307 PNP 25 33 kilohms 27 BC 238 NPN 28 5.6 kilohms 30 4.7 kilohms 314.7 kilohms The BC 237 NPN transistor is a general 5 lo 15 20 25 30 35 40 45 50 55 60 65 purpose silicon transistor having a maximum rated VCeo of 45 volts, and a maximum rated VebO of 5 volts. The BC 307 transistor is a general purpose PNP silicon transistor also having a maximum rated VCeo of 45 volts, and a maximum rated Vebo of 5 volts.The BC 238 transistor is a general purpose NPN silicon transistor having a maximum rated VCeo of 20 volts and a maximum rated Vebo of 5 volts.

If desired, a circuit which differs from that of Fig. 2 only in having a negative 12 volt supply and transistors of the opposite type to that shown in each case can be constructed. In such an exampie, the switching circuit embodying the invention would have two PNP transistors as the transistors 17 and 24, and an NPN transistor as the transistor 21.

Referring again to Fig. 2 as shown, in the example using the component values given above, the potential difference across the forward biased junction of the transistors,17, 21 and 24 is 0.6 volts. Consequently, when the input terminal 12 of the head 11 is connected to ground potential by the switching circuit embodying the invention, the respective voltages of the bases of the three transistors 17, 21 and 24 relative to ground potential are +0.6 volts, +11.4 volts and +0.6 volts, the collector and emitter electrodes of the transistors 17 and 24 being all substantially at ground potential, and the collector and emitter electrodes of the transistor 21 being at substantially + 12 volts.The fact that the collector electrodes of the transistor 17 is substantially at ground potential constitutes the provision of a low offset voltage at the input terminal 12 of the head 11 when the tape recorder is operating in its playback mode. This is important because the amplifier 26 provides a high level of amplification and changes of DC level at the input terminal 12 of the head 11 produce amplified level changes at the output of the amplifier 26. The audio frequency and bias frequency signals applied at the terminal 14 have a DC level at ground potential and so the change from record to playback and vice versa effected by operation of the switching circuit does not produce any objectionable effect at the output of the amplifier 26.

Fig.3 shows a switching circuit embodying the present invention but differing from the example shown in Fig.2 in having a relatively large offset voltage at the switching terminal when the switching circuit is in its closed or "on" condition.

The typical open circuit offset voltage for this example is 14 millivolts. The typical "on" resistance is 40 ohms.

In the switching circuit of Fig.3, a switching terminal 40 is connected to the collector electrode of a PNP transistor 41 which has its emitter electrode connected to the cathode of a semiconductor diode 42, the anode of which is connected to ground. The base of the transistor 41 is resistance coupled by a resistor 43 to the collector electrode of an NPN transistor 44 having its emitter electrode connected to the fixed contact 45 of a control switch 46 having its moveable contact 47 connected to a supply (not shown) of~12 volts. The base electrode of the transistor 44 is resistance coupled to ground by a resistor 48, and the fixed contact 45 of the control switch 46 is also resistance coupled to ground by a resistor 49.

When the switching circuit of Fig.3 ìs in the state shown in Fig.3, i.e., with the moveable contact 47 of the control switch 46 in contact with the fixed contact 45 so that the~12 volts of the supply is connected to the emitter electrode of the transistor 44, the switching circuit is in its "on" condition. It will be seen from Fig.3 that when the circuit is in this condition, the emitterbase junction of the transistor 44 is forward biased so that the transistor 44 presents a low impedance between its collector and emitter electrodes, and that as a result of the low impedance state of the transistor 44, the emitterbase junction of the transistor 41 and the diode 42 are aiso forward biased.If the diode 42 is a silicon diode, the voltage at its cathode when the diode is forward biased is approximately~.6 volts. Consequently, the voltage at the switching terminal 40 is approximately 0.6 volts relative to ground. Furthermore, if the transistors 41 and 44 are silicon transistors, the voltage drops across their respective emitter-base junctions when forward biased are approximately 0.6 volts.

Consequently, in the "on" condition of the switching circuit of Fig.3, when the diode 42 and the transistors 41 and 44 are silicon devices, the voltages relative to ground at the base electrodes of the transistors 41 and 44 are, respectively, ~1.2 volts and~11.4 volts, approximately. The resistors 43 and 48 must be chosen accordingly.

If the diode 42 is a germanium diode, the voltage drop across the diode when forward biased is approximately 0.2 volts and this value of voltage will appear as the offset voltage at the switching terminal 40.

Another embodiment similar to that of Fig.3 can be constructed in which the transistor 41 is an NPN transistor, the transistor 44 is a PNP transistor, the diode 42 is poled in the opposite direction from that shown in Fig.3, and the supply voltage is + 12 volts. The offset voltage at the switching terminal in the "on" state of the circuit is then positive, the value again depending upon whether the diode is a silicon diode or a germanium diode.

In the "off" condition of the embodiment of Fig.3, the control switch 46 is open so that the emitter of the transistor 44 is coupled to earth potential through the resistor 49. If a positive voltage relative to ground now appears at the switching terminal 40, the collector-base junction of the transistor 41 is forward biased but the collector-base junction of the transistor 44 is reversed biased and the diode 42 is reversed biased. Consequently, the positive voltage appearing at the switching terminal 40 is blocked by the diode 42 and the collector-base junction of the transistor 44, despite the transistor 41 being 5 10 15 20 25 30 35 40 45 50 55 60 in a low impedance condition.

When the switch 46 is open and a negative voltage appears at the switching terminal 40, the collector-base junction of the PNP transistor 41 is reversed biased and the emitter and base electrode of the transistor 41 remain at ground potential since the leakage current flowing to the collector electrode of the transistor 41 is insufficient to cause significant forward biasing of the emitter-base junction of the transistor 41 for that transistor to switch into its low impedance state.

The allowable excursions of voltage at the switched terminal 40 relative to ground are thus determined by the collector-base break-down voltages of the transistors 41 and 48 and the ' break-down voltage of the diode 42.

The operation of an embodiment differing from the switching circuit of Fig.3 only in having transistors of the opposite type, the diode reversed in polarity, and a positive voltage supply, is the same except that the polarities of the voltages occurring at the various points in the circuit are of the opposite sign.

Embodiments such as the switching circuit of Fig.3 which have a diode instead of a third transistor are more vulnerable in their "off" condition than the embodiments having three transistors since in this condition, when the voltage at the switched terminal is such as to reverse bias the collector-base junction of the first transistor, the current path from the base of the first transistor to ground through the emitter of that transistor does not contain a reverse biased PN junction.

The circuits of Figs.2 and 3 may be modified by replacing the resistors 31 and 49 with arrangements which connect the emitter of the second transistor directly to ground when the switching circuit is in its "off" condition. Such an arrangement is shown in Fig. 4.

Fig.4 shows the second transistor 50, which in this case is a PNP transistor, of a switching circuit embodying the present invention, the base electrode of the second transistor being resistively coupled to ground by a resistor 51 and the emitter electrode of the transistor 50 being connected to one fixed contact 52 of a two-pole two-way switch 53 having three further fixed contacts 54,55 and 56 and two moveable contacts 57 and 58. The moveable contacts 57 and 58 are ganged together as indicated in Fig. 4.

The fixed contacts 52 and 56 are permanently connected directly together, and so also are the fixed contacts 54 and 55. The moveable contact 57 is connected to a supply (not shown) of positive voltage and the moveable contact 58 is connected to ground. The switch is such that the moveable contacts 57 and 58 are either respectively in contact with the fixed contacts 55 and 56 or respectively in contact with the fixed contacts 52 and 54. When the moveable contact 57 is in contact with the fixed contact 55 and the moveable contact 58 is in contact with the fixed contact 56, as shown in Fig.4, the switching circuit (not completely shown in Fig.4) is in its "off" condition, since the emitter electrode of the transistor 50 is connected directly to ground.

When the moveable contact 57 is in contact with the fixed contact 52 and the moveable contact 58 is in contact with the fixed contact 54, the switching circuit is in its "on" condition, the positive voltage supply being connected directly to the emitter electrode of the transistor 50 so that the emitter-base junction of this transistor is forward biased.

The arrangement of Fig.4 can be further modified so as to switch a resistor corresponding to the resistors 31 and 49 between the emitter electrode of the transistor 50 and ground only when the switching circuit is in its "on" condition.

This is achieved by connecting a suitable resistor between the emitter electrode of the transistor 50 and ground. It will be seen from Fig.4 that when this is done, the resistor thus connected is in parallel with the current path through the emitterbase junction and the resistor 51 when the moveable contact 57 is in contact with the fixed contact 52, and that this additional resistor is short circuited by the direct connection between the fixed contacts 52 and 56 when the moveable contact 58 is in contact with the fixed contact 56.

Although all the transistors mentioned in the examples described hereinbefore with reference to the accompanying drawings are silicon transistors, germanium transistors can be used instead in which case the emitter to base voltage for each transistor when the emitter to base junction is forward biased is approximately 0.2 volts.

Embodiments of the invention may be constructed of discrete components or may be made in the form of an integrated circuit.

Furthermore, in either of these forms of embodiment, the control switch may be an electronic switch instead of a mechanical switch.

Claims (4)

Claims

1. A switching circuit including a switching terminal connected to the collector of a first junction transistor, a control switch having one terminal thereof connected to the emitter of a second junction transistor of the opposite type to the first junction transistor, the collector of the second transistor being resistance coupled to the base of the first transistor, the base of the second transistor being resistance coupled to a common terminal, and the emitter of the first transistor being coupled to the common terminal through a .semiconductor device including a junction poled in the same direction as the emitter-base junction of the first transistor.

2. A circuit according to claim 1, wherein the said semiconductor device is a third junction transistor which has its base resistance coupled to the collector of the second transistor.

3. A circuit according to claim 2, wherein the emitter of the third transistor is connected to the emitter of the first transistor.

4. A switching circuit substantially as described hereinbefore with reference to any one of Figs.2,3 and 4 of the accompanying drawings.