The zero gain amplifier (Q1, R2) and stabilized current source (Q2, R3) from activities 7 and 8 can be used in conjunction with a PNP current mirror stage (Q3,Q4) in negative feedback to build a circuit which provides a constant or regulated output voltage over a range of input voltages.
ADALM2000 Active Learning Module
1 - 2.2KΩ Resistor ( or any similar value )
1 - 100Ω resistor
2 - small signal NPN transistors (2N3904 or SSM2212)
2 - small signal PNP transistors (2N3906 or SSM2220)
The breadboard connections are as shown in the diagram below. The output of the AWG1 drives the emitters of both PNP transistors Q3 and Q4. Q3 and Q4 are wired as a current mirror with their bases connected together with the collector of Q3. The collector of Q4 connects to resistor R1. Resistors R1, R2 and transistor Q1 are connected as in the previous zero gain amplifier section. Since the VBE of Q2 is always smaller than the VBE of Q1, You should select Q1 and Q2 from your inventory of devices such that (at the same collector current) Q2's VBE is less than Q1's VBE. The base of transistor Q2 is connected to the zero gain output at the collector of Q1. The collector of Q2 connects to the input side of the PNP current mirror at the base - collector of Q3. The 2+ (Single Ended) scope input is used to measure the output voltage at the collector of Q4.
Figure 1 Regulator Version 1
Figure 2 Regulator Version 1 Breadboard Circuit
Waveform generator 1 should be configured for a 1 KHz triangle wave with 4 volt amplitude and 2V offset. The Single ended input of scope channel 2 (2+) is used to measure the stabilized output voltage at the collector of Q4 ( negative inputs 1= and 2- should be connected to ground).
Plot the output voltage (as measured at the collector of Q4) vs. the input voltage. At what input voltage level does the output voltage stop changing i.e. regulate? This is called the “drop out” voltage. For input voltages above the drop out voltage, how much does the output voltage change for each volt of change at the input? The change in Vout / change in Vin is called line regulation. Connect a variable resistor from the output node to ground. With the input voltage fixed (i.e. connected to the fixed Vp board power supply), measure the output voltage for various settings of the resistor. Calculate the current in the resistor for each setting. How does the output voltage vary vs. output current? This is called load regulation.
Figure 3 Regulator Version 1 Waveform
The problem with the circuit in regulator version 1 is that the current available to an output load is limited by the feedback current supplied from NPN Q2 mirrored through PNPs Q3 and Q4. It would be desirable to build a circuit which provides a constant or regulated output voltage over not only a range of input voltages but also output load currents. This second circuit utilizes an emitter follower output stage to provide the current to the output.
1 - 2.2KΩ Resistor
1 - 100Ω resistor
1 - 10KΩ variable resistor (potentiometer)
1 - 4.7KΩ resistor (resistors can any similar value selected for desired circuit operation)
4 - small signal NPN transistors (2N3904 and SSM2212)
The breadboard connections are as shown in the diagram below. As before transistor Q1 and resistors R1 and R2 are configured as a zero gain amplifier. Transistor Q2 and variable resistor R3form a stabilized current source. If the SSM2212 matched NPN pair is used it is best that it be used for devices Q1 and Q2. Common emitter stage Q3 along with its collector load R4 provide gain. Emitter follower Q4 drives the output node and closes the negative feedback loop.
Figure 4 Regulator Version 2
Figure 5 Regulator Version 2 Breadboard Circuit
Waveform generator W1 should be configured for a 1 KHz triangle wave with 4 volt amplitude and 2V offset. Scope channel 2 (2+) is used to measure the stabilized output voltage at the emitter of Q4.
Repeat the drop out voltage, line and load regulation measurements for this circuit. How are they different than the first regulator circuit?
Figure 6 Regulator Version 2 Waveform
ADI Mini Tutorial on Voltage references
The CA3045,46 ( LM3045, 46 ) NPN transistor array is a good alternate choice for building this example circuit. See pinout below.
All the emitters can be tired to ground ( pins 3,7,10,13 ). Devices Q1, Q2 and Q3 can be connected in parallel and serve as Q2 in figure 2. Q4 and Q5can be used for Q1 and Q3in figure 2. An individual device such as a 2N3904 etc. can be used for Q4 in figure 2. The 3 to 1 emitter area ratio will result in an output voltage very nearly 1.2 volts if R1 and R3 are both equal to 2KΩ (when R2 is 100Ω).
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