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.
As in all the ALM labs we use the following terminology when referring to the connections to the M1000 connector and configuring the hardware. The green shaded rectangles indicate connections to the M1000 analog I/O connector. The analog I/O channel pins are referred to as CA and CB. When configured to force voltage / measure current -V is added as in CA-V or when configured to force current / measure voltage -I is added as in CA-I. When a channel is configured in the high impedance mode to only measure voltage -H is added as CA-H.
Scope traces are similarly referred to by channel and voltage / current. Such as CA-V , CB-V for the voltage waveforms and CA-I , CB-I for the current waveforms.
ADALM1000 hardware module
1 - 2.2 KΩ Resistor ( or any similar value )
1 - 100 Ω resistor
3 - small signal NPN transistors (2N3904)
3 - small signal PNP transistors (2N3906)
The breadboard connections are as shown in figure 1. The output of the channel A voltage generator, CA-V, 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 CB-H scope input is used to measure the output voltage at the collector of Q4.
Figure 1 Regulator Version 1
Channel A voltage generator, CA-V, should be configured for a 100 Hz triangle wave with 5 volt Max and 0V Min values. The scope channel B is set to Hi-Z mode and CB-H is used to measure the stabilized output voltage at the collector of Q4.
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 +5 V 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.
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 - 2.2 KΩ Resistor
1 - 100 Ω resistor
1 - 10 KΩ variable resistor (potentiometer)
1 - 4.7 KΩ resistor (resistors can be any similar value selected for desired circuit operation)
4 - small signal NPN transistors (2N3904 and SSM2212)
The breadboard connections are as shown in figure 2. 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 2 Regulator Version 2
The Channel A voltage generator, CA-V, should be configured for a 100 Hz triangle wave with 5 volt Max and 0 V Min values. As in version 1 the scope channel CB-H, in Hi-Z mode 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?
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 2 K? (when R2 is 100?).
For Further Reading:
Return to Lab Activity Table of Contents