“The zero-gain amplifier (Q1, R2) and regulated current source (Q2, R3) from the previous experiments can be used in conjunction with the PNP current mirror stage (Q3, Q4) in negative feedback to build a range of input voltages that provide circuit with constant or adjustable output voltage.
“
By: Doug Mercer, Consulting Researcher, Analog Devices; Antoniu Miclaus, Systems Applications Engineer
The purpose of this lab is to build and study several types of references/regulators and is divided into the following sections:
► Adjustable voltage reference
► Enhanced voltage reference
► Shunt Regulators
Regulated Voltage Reference
Target
The zero-gain amplifier (Q1, R2) and regulated current source (Q2, R3) from the previous experiments can be used in conjunction with the PNP current mirror stage (Q3, Q4) in negative feedback to build a range of input voltages that provide circuit with constant or adjustable output voltage.
Material
► ADALM2000 Active Learning Module
► Solderless Breadboard
► A 2.2 kΩ resistor (or other similar value)
► A 100 Ω resistor
► Two small signal NPN transistors (2N3904 or SSM2212)
► Two small signal PNP transistors (2N3906 or SSM2220)
instruction
The breadboard connections are shown in Figure 1. The output of AWG1 drives the emitters of PNP transistors Q3 and Q4. Q3 and Q4 are configured as current mirrors with their bases connected to Q3’s collector. The collector of Q4 is connected to resistor R1. Resistors R1 and R2 and transistor Q1 are connected as shown in the November 2020 Student Zone Experiment, “ADALM2000 Experiment: Zero-Gain Amplifier.” Since Q2’s VBEalways less than the V of Q1BEso Q1 and Q2 should be selected from the device inventory such that (at the same collector current) the V of Q2BEless than the V of Q1BE. The base of transistor Q2 is connected to the zero gain output of the collector of Q1. The collector of Q2 is connected to the inputs of the PNP current mirror, the base and collector of Q3. The 2+ (single-ended) oscilloscope input is used to measure the output voltage on the collector of Q4.
Figure 1. Regulator circuit.
hardware setup
Waveform Generator 1 should be configured as a 1 kHz triangle wave with a peak-to-peak amplitude of 4 V and an offset of 2 V. The single-ended input (2+) of oscilloscope channel 2 is used to measure the regulated output voltage on the collector of Q4 (negative inputs 1- and 2C should be grounded).
Figure 2. Regulator breadboard circuit.
procedural steps
Plot the output voltage (measured at the collector of Q4) versus the input voltage. At what input voltage level does the output voltage stop changing/regulating? This is called differential pressure. For input voltages higher than dropout, how much does the output voltage change for every volt change in the input voltage? VOUTchange/VINThe change is called voltage regulation. Connect the output node of the variable resistor to ground. With the input voltage fixed (i.e. connected to a fixed Vp board power supply), measure the output voltage for various settings of the resistor. Calculate the current in the resistance for each setting. How does the relationship between output voltage and output current change? This is called load regulation.
Figure 3. Regulator oscilloscope XY plot.
Enhanced Voltage Reference
Target
The problem with the regulator circuit in the previous section is that the current available to the output load is limited by the feedback current provided by NPN Q2 mapped through PNP Q3 and Q4. We want to build a circuit that provides a constant or adjustable output voltage not only over the input voltage range, but also over the output load current range. This second circuit utilizes an emitter follower output stage to supply current to the output.
Material
► A 2.2 kΩ resistor
► A 100 Ω resistor
► One 10 kΩ variable resistor (potentiometer)
► A 4.7 kΩ resistor (can be any similar value resistor chosen for desired circuit operation)
► Four small signal NPN transistors (2N3904 and SSM2212)
instruction
The breadboard connections are shown in Figure 4. Transistor Q1 and resistors R1 and R2 are still configured as zero-gain amplifiers. Transistor Q2 and variable resistor R3 form a stable current source. If using the SSM2212 matched NPN pair, it is best to use it for devices Q1 and Q2. Common emitter output stage Q3 and its collector load R4 provide gain. Emitter follower Q4 drives the output node and closes the negative feedback loop.
Figure 4. Enhanced voltage regulator.
hardware setup
Waveform generator W1 should be configured as a 1 kHz triangle wave with a peak-to-peak amplitude of 4 V and an offset of 2 V. Oscilloscope channel 2 (2+) is used to measure the regulated output voltage on the emitter of Q4.
Figure 5. Enhanced regulator breadboard circuit.
procedural steps
Repeat the measurement of dropout, line, and load adjustment for this circuit. How are they different from the first regulator circuit?
Figure 6. Enhanced regulator waveform XY plot.
Shunt Regulator
Target
A zero gain amplifier (Q1, R2) and a regulated current source (Q2, R3) can be used with a common emitter amplifier stage (Q3) in negative feedback to build a constant or adjustable output over a range of input currents voltage 2-port circuit.
Material
► ADALM2000 Active Learning Module
► Solderless Breadboard
► Jumper
► A 2.2 kΩ resistor (or other similar value)
► A 100 Ω resistor
► A 1 kΩ resistor (or similar)
► One 10 kΩ variable resistor (potentiometer)
► Three small signal NPN transistors (2N3904 and SSM2212)
instruction
The breadboard connections are shown in Figure 7. The output of the function generator drives one end of resistor R4. Resistors R1 and R2 and transistor Q1 are connected as shown in the November Student Zone article, “ADALM2000 Experiment: A Zero-Gain Amplifier (BJT).” Resistor R3 and transistor Q2 are connected as shown in the January 2021 article, “ADALM2000 Experiment: Regulated current source.” If using the SSM2212 matched NPN pair, it is best to use it for devices Q1 and Q2. Add Q3, connect its emitter to ground, base to Q2’s collector, and collector to the node combining R1, R3, R4 and scope input 2+.
Figure 7. Bandgap shunt regulator.
hardware setup
Waveform generator W1 should be configured as a 1 kHz triangle wave with a peak-to-peak amplitude of 4 V and an offset of 2 V. The single-ended input (2+) of oscilloscope channel 2 is used to measure the regulated output voltage on the collector of Q3.
Figure 8. Bandgap shunt regulator breadboard circuit.
procedural steps
Configure the oscilloscope to capture multiple cycles of the two measured signals. Make sure the XY function is enabled. Figure 9 provides an example of an image displayed by an oscilloscope. When adjusting the variable resistor R3, observe the adjustment of the output voltage.
You can find answers to your questions on the Student Zone blog.
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