The objective of this activity is to investigate the ΔVBE concept to produce an output current which is stabilized (less sensitive) to variations of the input voltage level. Feedback is used to build a circuit which produces a constant or regulated output current over a range of supply voltages
The AD590 is a 2-terminal integrated circuit temperature transducer that produces an output current proportional to absolute temperature. The schematic diagram of this floating current source is shown in figure 7 of the AD590 datasheet. The a simplified and less accurate version of this concept is used as the experimental circuit in this activity.
The AD590 uses a fundamental property of silicon BJT transistors to realize its temperature proportional characteristic. If two identical transistors are operated at a constant ratio of collector current densities, r, then the difference in their base-emitter voltage is (kT/q)(In r). Because both k (Boltzman’s constant) and q (the charge of an electron) are constant, the resulting voltage is directly proportional to absolute temperature (PTAT). For more details please refer to the datasheet.
ADALM2000 Active Learning Module
1 - 500Ω Variable Resistor, Potentiometer
1 - 100Ω Resistor
3 - small signal NPN transistors (2N3904)
3 - small signal PNP transistors (2N3906)
Build the circuit shown in figure 1 on your solder-less breadboard. The green boxes indicate where to connect the ADALM2000. PNP transistors Q1, Q2 and Q3 form a current mirror with a gain of two, the output current is twice the input current. NPN transistors Q4, Q5 and Q6 along with variable resistor R1 form the ΔVBE part of the circuit. Resistor R2 is used to measure the current flowing in the circuit ( scope channel 2 ) as the voltage across the circuit changes ( scope channel 1 ).
Figure 1 Floating current source (as a sink tied to a negative supply)
The output current is set by R1. The difference in VBE ( ΔVBE ) between Q4 and the parallel combination of Q5,Q6 appears across R1. The PNP mirror, Q1,Q2 and Q3, has a gain of 2, if we assume they are of identical size. Thus the current in Q4 is twice the combined current of Q5 and Q6. Again if we assume Q4, Q5 and Q6 are also identical in size, the current density ratio is 4 and the difference in VBE will be:
Because of the absolute temperature term in this equation the current will be proportional to absolute temperature. This can be a useful characteristic in certain instances but also undesirable in others.
Figure 2 Floating current source (as a sink tied to a negative supply) Breadboard Circuit
Configure waveform generator AWG1 as a triangle wave with a frequency of 100 Hz and an amplitude of 10 V peak-to-peak with 0 V offset. The scope display should be set in both voltage vs. time and in XY mode with channel 1 on the horizontal axis and channel 2 on the vertical axis. Be sure to turn on the power supply only after you have completed and double checked your connections.
What is the minimum voltage that the current source needs across it to maintain a more or less constant current?
Is it larger or smaller than 2*VBE and why?
Measure the ΔVBE across adjustable resistor R1with AWG1 set to a fixed voltage. How does the value of ΔVBE change as the resistance of R1 is varied?
In figure 1 we referenced the negative end of the circuit to a negative power supply. To prove that this circuit is truly a floating current source, rearrange your breadboard to look like figure 5 and repeat your measurements.
Figure 4 Floating current source (as a source tied to a positive supply)
Is there any measurable difference in the current vs. voltage characteristics for the circuit used as a current sink vs. a current source?
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