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Single PN temp sensor performs multiple functions

15 Jan 2015  | Petre Petrov

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Transistors such as BC550, BC549, etc. have a guaranteed temperature coefficient of the voltage base-emitter (TKUbe) of the transistor. In order to produce good temperature stability we can use two transistors from the same type as T1 and T2.

One transistor, such as T1 will generate the output current and the other transistor (T2 in this case), will produce reference voltage with appropriate TKUbe. In this manner we will obtain the next circuit of the CCG.

Figure 1d is modified from figure 1b. This is the circuit of CCG with stabilisation of the base voltage with transistor working as diode and a resistor R2. The transistors T1 and T2 should be from the same type and with approximately the same temperature coefficient TKUbe.

We can go further and to produce better reference voltage for the base-emitter voltage of T1. Figure 1 e is a circuit of CCG with stabilisation of the base-voltage of T1 with the transistor T2 working as diode and a reference diode D1. D1 can be for any appropriate voltage, e.g. 1.2V, 2.5V, 4.096V, etc. and we can neglect the temperature coefficient of D1. We can use the reference diodes and ICs as LM385-1.2V, ADR512 (1.2V), LM113 (1.2V), LT1004 (1.2V), LM431A (2.5V), and similar or better. For example, the minimal working current for LM385-1.2V is 10uA and the maximal working current is 20mA, which fits well for many simple CCGs.

Sometimes we may wish to monitor the base current and the emitter current of the output transistor in the CCG. In this case we are in need of one more resistor in the series of the base of T1 and two analogue inputs of the embedded ADC.

Figure 1f presents the circuit CCG with a possibility for measurement of the base current and the voltage of the emitter of the transistor T1. The voltage drop over the resistor R4 is proportional to the base current of T1. The voltage drop over the resistor R3 is proportional to the emitter current Ie which is practically equal to the collector current Ic. The measurements of these two voltages are useful because they give an idea about the gain and status of the transistor T1.


Figure 1: Circuit of the constant current generator (CCG) with transistor T1, temperature compensated reference source (D1 and T2) and ON/OFF switch with open collector or open drain.


Practical implementation
Figure 2 presents the constant current generator (CCG) with transistor T1, which is derived from the circuit from figure 1e. The base of the transistor T1 is connected to the voltage reference voltage built around the reference diode D1 and the junction emitter-base of the transistor T2. In this case the reference voltage Vref (T1) is calculated with the formula:


Vref (T1) = Vz(D1) + Vbe(T2)


or


Vref (T1) = 1.2V + 0.65V = 1.85V


D1 is reference diode for 1.2V as LM385-1.2. We may use also LM385-2.5 or LM431 but they will produce the reference voltage of 2.5V which can be too much when the power supply Vcc1 is low.

The important element here is transistor T2. It executes the following main functions:

 • T2 work as temperature sensor with a negative temperature coefficient. If we use BC549, BC550 and similar transistors we will have temperature coefficient TKU of -2mV/C (according to an NXP data sheet).
 • T2 work as reference voltage source together with D1.

If the transistors T1 and T2 are from the same type with guaranteed TKUbe the voltage drop over both junction base emitter will be practically equal and the TKUbe of both junctions will be approximately the same.

In order to have full temperature compensation T1 and T2 should be in good thermal contact. If we do not have that contact between T1 and T2 we can measure the ambient temperature with T2. T1 and T2 are high gain transistors from the same type. That is why we can accept that the emitter currents are equal to the corresponding collector currents or:


Ie(T1) = Ic(T1)


and


Vbe(T2) = Vbe(T1)


also


Ic(T1) = Ie(T1) = ( V(D1)+ Vbe(T2) – Vbe(T1))/R1 = 1.2V/62Ohm = 20mA (approx).

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