GB2158312A

GB2158312A - Transistor circuits

Info

Publication number: GB2158312A
Application number: GB08410948A
Authority: GB
Inventors: John William Shadrach
Original assignee: Smith Meters Ltd
Current assignee: Smith Meters Ltd
Priority date: 1984-04-28
Filing date: 1984-04-28
Publication date: 1985-11-06
Also published as: GB8410948D0

Abstract

A silicon junction transistor TR1 is intended to change state when a predetermined voltage VTH appears at the upper end of R1. A negative coefficient resistor R2 is connected across the base/emitter terminals of the transistor in order to compensate for the variations of the transistor switching voltage with temperature. Thus the voltage VTH at which the transistor switches has reduced variation with temperature. Switching is effected when the voltage of a battery 13 supplying a load 12 falls below a predetermined level to turn off TR2, thus disconnecting the load. <IMAGE>

Description

SPECIFICATION Transistor circuits This invention relates to transistor circuits in which a transistor is required to change state at a nominal potential difference. The actual point at which the changeover takes place is liable to change with change of environmental temperature so that the actual changeover potential difference may vary from the design value.

The invention provides a transistor circuit having a transistor selected to change state on being subjected to a nominal potential difference across two of its terminals, and a resistance of the kind providing a negative temperature coefficient connected across said terminals, whereby to adjust the potential difference applied across said terminals in the sense to compensate for temperature variations of the transistor characteristic.

The resistance may be a combination of fixed resistance and a thermistor.

According to another aspect the invention provides a method of compensating for the temperature variations of a transistor by comparing the characteristic of negative temperature coefficient resistances with that of the transistor, selecting a resistance having a characteristic which provides compensation for the transistor and connecting said resistance across the base/emitter terminals of ehe transistor.

Specific embodiments of the invention are shown in the accompanying drawings, in which: Figure 1 is a representative circuit, Figure 2 is a curve of a thermistor temperature characteristic, and Figure 3 is another circuit incorporating the invention.

Referring first to Figs. 1 and 2, an electric circuit as shown in Fig. 1 is incorporated in a larger circuit (not shown). A silicon junction transistor T, is intended to change state when a predetermined voltage VTH appears on line (11).

The base and emitter of the transistor T, are connected at points A and B across a resistor R2 and through resistor R1 to the line (11). The potential difference VAB between points A and B which causes the transistor to change state is related to VTH by:

R2 may for instance be 22 kilohms.

The potential difference at which the transistor changes state varies with temperature, for instance it may vary by -2mv/"C, because the base emitter connections of the transistor are a junction device, the forward volt drop of which is temperature dependent. The value of VAB therefore drops as the temperature increases. For transistor T1, VAB is 0.71 volts at 0 C and 0.61 volts at 50"C. Thus without temperature compensation the circuit operates at different values of VTH, according to the ambient temperature.

To provide a measure of compensation, resistor R2 is a negative coefficient resistor (a thermistor). The resistance drops as the temperature increases, thus causing the voltage at point A to drop for any given value of VTH. A general formula for the resistance of a thermistor is: R = AeR/T where A and B are constants.

T = absolute temperature.

The thermistor selected has a resistance of 200 kilohms at 0 C dropping to 25 kilohms at 50"C.

Fig. 2 shows the resistance/temperature curve of the selected thermistor.

Using these values it is found that the values of VTH at which the transistor changes state are 3.51 volts at O"C and 3.54 volts at 50"C. A similar circuit in which R2 is not a thermistor but is fixed at 5.6 kilohms gives values for VTH of 3.5 volts at 0 C and 3.0 volts at 50"C. It will be seen therefore that by proper selection of the thermistor for the expected temperature range, the operation of the circuit is made considerably less temperature sensitive. This is achieved by comparison of the thermistor resistance variation with that of the transistor so as to achieve the best available match.

It will be noted that the resistance characteristic does not have the same shape as the transistor characteristic so perfect match is not possible. The best match usually has two matching points with a small variation over the rest of the temperature range.

Fig. 3 comprises an integrated circuit (12), which has a predetermined operating voltage range, and is powered by a battery (13). As the battery discharges, its output voltage falls and it becomes necessary to disconnect the battery from the integrated circuit in order to avoid operation below the correct voltage range. Such operation can cause the integrated circuit to enter a state from which it does not recover even when the battery voltage is increased into the correct range.

Transistor TR1 changes state and ceases to conduct when its base/emitter voltage drops to a predetermined level related to the iower end of said predetermined operating voltage range.

When transistor TR, stops conducting, it renders transistor TR2 (connected in the line from the battery to the integrated circuit) non-conducting, thereby effectively disconnecting the battery. In the absence of the present invention the changeover voltage for transistor TR, is temperature dependent, so that a battery of greater capacity and/or voltage is required to be used to offset the temperature effect and avoid premature cutting off of the battery power. However, resistance R2 is selected to be a combination of a fixed resistance and a negative temperature coefficient resistor (a thermistor). By matching the characteristics of R2 with the temperature characteristics of the base/emitter junction of transistor TR1, the circuit's temperature dependence is very much reduced, as described in relation to Fig. 1. Thus a more economical battery can be used to drive the integrated circuit.

Claims

1. A transistor circuit having a transistor selected to change state on being subjected to a nominal potential difference across two of its terminals, and a resistance of the kind providing a negative temperature coefficient connected across said terminals, whereby to adjust the potential difference applied across said terminals in the sense to compensate for temperature variations of the transistor characteristic.

2. A transistor circuit as claimed in claim 1, wherein said transistor is a silicon junction transistor and the said two terminals are the base and emitter.

3. A transistor circuit as claimed in claim 2, wherein a variable switching voltage is applied across the base and emitter of the transistor so that the transistor ceases to conduct when the switching voltage drops below a switching value, which switching value is compensated for temperature variations by the operation of said resistance.

4. A transistor circuit as claimed in claim 2 or claim 3, wherein the resistor has a resistance of 200 kilohms at 0 C dropping to 25 kilohms at 50"C while the transistor has a temperature characteristic of - 2mv/'C.

5. A transistor circuit as claimed in any of claims 1 to 4, connected in parallel with a battery-powered integrated circuit, so that the battery is disconnected when its voltage output drops below a predetermined value.

6. A transistor circuit as claimed in any of claims 1 to 5, wherein said resistance is a combination of a fixed resistance and a thermistor.

7. A transistor circuit substantially as described hereinbefore with reference to Fig. 1 or Fig.

3 of the accompanying drawings.

8. A method of compensating for the temperature variations of a transistor by comparing the characteristics of negative temperature coefficient resistances with the temperature characteristic of the transistor, selecting a resistance having a characteristic which provides compensation for the temperature characteristic of the transistor, and connecting the resistance across the base/emitter terminals of the transistor.