The objective of this lab activity is to investigate ways to produce negative reference voltages. Positive voltage references or regulator configurations are more commonly available. Conventional methods of generating a negative reference voltage from a positive voltage involve inverting op-amp stages which tend to rely on precision matched resistors for accuracy.
In figure 1(a) the simple zener diode circuit, consisting of RZ and DZ from the zener diode regulator lab activity, is used to produce a positive reference voltage, +VREF. In a positive voltage reference a non-inverting op-amp buffer is often included to scale the output voltage and supply any current needed in the load. The obvious method for generating a negative reference voltage is to instead use an inverting op-amp stage as shown in the figure. This approach requires two precision matched resistors, R1 and R2. Errors in the matching, in addition to any offset voltage in the op-amp, will produce errors at the negative output -VREF. However, one potential side benefit of this inverting amplifier configuration is that -VREF need not have the same absolute value as +VREF. The negative reference voltage can be scaled up or down by altering the ratio of R1 and R2. The alternate configuration we will be investigating in this lab activity is shown in figure 1(b). It generates a negative reference voltage without the dependence on ratio matched resistors, potentially providing higher accuracy with fewer components.
Figure 1 Generating a negative voltage reference
By examining figure 1(a) we see that, by the virtual ground nature of the inverting op amp configuration, the zener voltage +VREF is impressed across resistor R1. If R2 is exactly equal to R1 this same voltage VREF will also appear across R2 but with the sign reversed with respect to ground. Since the voltage across R2 is the same as that across the zener diode we can in effect replace R2 with the diode in the feedback loop as in figure 1(b) and still produce the same voltage at -VREF. RZ simply sets the bias current level in the zener much as RZ in 1(a). In 1(b) IZ is equal to VDD/RZ where in 1(a) IZ is equal to (VDD - +VREF)/RZ. To design for the same IZ in both cases we simply change the value of RZ. Capacitor C1 decouples the reference diode between its ground and output terminals. In addition low inductance 0.1 µF supply decoupling capacitors (not shown in the figure) are often connected to +VDD and -VSS very close to the op-amp.
In theory this circuit can be built using almost any three terminal voltage reference circuit and a low noise, low offset operational amplifier. The lab activities on band-gap reference circuits    use NPN transistors to build positive voltage references. To build a negative reference based on the band-gap concept we would require high quality PNP transistors and the PNPs generally available in IC processes are not as high quality as the available NPN devices. These NPN based band-gap circuits will provide a couple of examples we can used to explore this negative reference configuration. The first circuit iteration in step 1 of this lab will use a diode as a reference and further iterations will substitute NPN transistor based two terminal ( shunt ) and three terminal ( series ) circuits as the reference element.
ADALM2000 Active Learning Module
Solder-less breadboard, and jumper wire kit
1 - 4.7 KΩ resistor
2 - 1.5 KΩ resistors
2 - 20 KΩ resistors
1 - 2.2 KΩ resistor
1 - 100 Ω resistor
1 - 10 KΩ variable resistor (potentiometer)
4 - small signal NPN transistors (2N3904 and SSM2212)
2 - LEDs (any color will do)
1 - OP482 or OP484 quad op-amp
1 - 1 nF Capacitor
2 - 0.01 uF Capacitors
2 - 0.1uF Capacitors ( supply decoupling capacitors for + and - 5 V supplies )
The zener diode ( 1N4735 ) supplied in the ADALP2000 Analog Parts Kit is a 6.1 volt diode. 6.1 volts is much too high a reverse breakdown voltage to build this circuit using the fixed +/- 5 volt power supplies of the ADALM2000 hardware. The forward voltage of an LED is in the range of 1.6 to 2.0 volts depending on the color of the diode. While not a proper reference diode, we can build the circuit for instructional purposes using the LEDs from the ADALP2000 Analog Parts Kit.
Build both of the versions of the circuits in figure 1(a) and 1(b) as shown in figure 2 on your solder-less breadboard. Use two LEDs preferably of the same color. Green LEDs will have a higher forward voltage drop than red or yellow. We want the diode current, ID, to be about 1 mA and the as close to this same value in both versions of the circuit. In the case (b) ID will be +5/R4 so a 4.7 KΩ resistor would give about 1 mA. In case (a) ID will be (+5-VD)/R3. If we use 2 V as an estimate for VD, then R3 would be around 3 KΩ. You can get 3 KΩ by connecting two 1.5 KΩ resistors from the Parts Kit in series. Also for case (a) we need to pick values for R1 and R2. We want the current in R1 to be much smaller than the current in R3. So setting R1 and R2 to a much higher value such as 20 KΩ should satisfy that condition.
Figure 2, LED based volt regulator example
Open the voltage supply control and the voltmeter instrument windows from the Scopy software. A DMM, if available, could be useful to more accurately measure the DC voltages in the circuit than the Scopy voltmeter instrument.
Figure 3 LED based volt regulator breadboard connections
Figure 4 Scopy voltmeter
What voltage did you measure at -VREFfor the circuits (a) and (b)? What voltage did you measure at the LED? Are these the correct expected values and why?
Modify your breadboard setup from step 1 as shown in figure 3. Be sure to turn off the power supplies before making any modifications to your breadboard. Replace the LED diode with the shunt regulator stage from earlier lab . Resistors R1, R2 and transistor Q1 are connected as the zero gain amplifier from the earlier lab . Resistor R3 and transistor Q2 are added as in the stabilized current source lab . If the SSM2212 matched NPN pair is used it is best that it be used for devices Q1 and Q2. Q3is added as common emitter stage, its base connected to the collector of Q2 and collector connected to the combined node of R1, R3 R4.
Figure 5 NPN shunt band-gap reference example
Turn on both the positive and negative power supplies. Observe the voltage at -VREF, pin 14 of the op amp and across the band-gap shunt regulator (collector and emitter of Q3) . You can adjust potentiometer R3 to produce a -1.25V reference voltage.
To test the headroom requirements for +VDD, disconnect the fixed positive power supply from +VDD and remove any supply decoupling capacitors. Be sure to turn off the power supplies before making any changes or additions to your breadboard. Now connect +VDD to AWG 1. Set AWG 1 to trapezium (trapezoid) waveform at 100 Hz. Set the amplitude to 5V peak-to-peak with a 2.5V offset for a 0 to +5V swing. Connect scope channel 1 to the output of AWG1 and connect scope channel 2 to -VREF of the first example circuit at pin 14 of the OP482. Use the oscilloscope instrument in the XY mode, scope channel for X and scope channel 2 for Y. Start AWG 1 and turn on the fixed negative 5V power supply. Record the minimum +VDD voltage where -VREF starts to remain constant at -1.25V.
To test the headroom requirements for -VSS, reconnect +VDD to the fixed positive power supply. Disconnect the fixed negative power supply from -VSS and remove any supply decoupling capacitors. Now connect -VSS to AWG 1. Set the amplitude to 5V peak-to-peak with a -2.5V offset for a 0 to -5V swing. Start AWG 1 and turn on the fixed positive 5V power supply. Repeat your measurements of pins 14 of the OP482 recording the lowest value for -VSS where the reference voltage is constant.
Modify your breadboard setup from step 1 as shown in figure 4. Be sure to turn off the power supplies before making any modifications to your breadboard. Change the two terminal, shunt, regulator used in step 2 to the three terminal reference  by adding emitter follower stage Q4, and compensation capacitor C1.
Figure 7 NPN three terminal band-gap reference example
Turn on both the positive and negative power supplies. Observe the voltage at -VREF, pin 14 of the op amp and across the band-gap three terminal regulator (emitter of Q4 and emitter of Q3).
Repeat the supply headroom tests you did in Step 2 for this configuration. Are there any differences?
 Activity: Zener Diode Regulator, EPS Activity: Zener Diode Regulator
 Activity 9. Regulated Voltage Reference
 Activity 10. Shunt regulator
 EPS Activity: The Band-Gap Voltage Reference
 Activity 7. Zero gain amplifier (BJT)
 Activity 8. Stabilized current source (BJT)
Return to Lab Activity Table of Contents