Single Supply Op-Amp Amplifiers Procedure
Instructional Objectives
1. Build a Split Supply Single Op-amp Differential Amplifier and determine the basic specifications.
2. Change to a Split Supply Non-Inverting Amplifier and determine the basic specifications.
3. Change to a Single Supply Non-Inverting Amplifier and learn what changes are needed to match
the specs of the split supply amplifier using impractical methods.
4. Relax the DC amp specification and maintain the AC amp spec still using impractical methods.
5. Create a single supply amp with 1 power supply resulting in a practical design.
Procedure
Parts needed: 4 –10K 1% resistors (blue), 4 – 20K 1% resistors (tan), 1 – 1.0uF film capacitors,
2 – 0.1uF ceramic caps. 1 –TLV271 op-amp.
1. Equipment: You will use the Oscilloscope with the Wag Gen output. You will use the Power
supply.
STOP: Go to the click the link below right now
and read: Why is there a black cable on the power supply. Read This!
2.
It explains how to setup the power supply for this lab!
3. Build the circuit shown in figure 1. A proto-board example for parts layout is shown in figure 2..
Figure 1: Differential Input Amplifier with Split Supply.
•
•
•
•
•
•
•
Connect the +30V power supply set to 5V to the capacitor lead on pin 7 (Red)
Connect the -30V power supply set to -5V to the cap lead on pin 4 (Blue). This will be set to -5V
automatically if the tracking is turned on.
Connect the ‘GND’ 2 terminals shorted by the short black cable of the power supply to the circuit
ground. Always do this.
Connect the CH1 scope probe to Vin1 and GND.
Connect the CH2 scope probe to Vout and GND.
Connect Wave Gen to Vin1 set to a 1KHz 1VPP sine wave.
Connect Vin2 to GND with a wire.
This happens to be the first Case in Table 1.
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Single Supply Op-Amp Amplifiers Procedure
Figure 2 is the scope traces with Vin and Vout of case 1 displayed with an visual offset a little so you
can see both traces.
Figure 2: Case 1 Differential Amplifier Vin and Vout traces.
Figure 2: Breadboard example of the Difference Amp with the Wav Gen on the positive input and the
negative input grounded.
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Single Supply Op-Amp Amplifiers Procedure
Red and Blue mini clips are +5 and -5 VDC. There is a somewhat hidden black wire that connects the
2 busses together for GND. The 499 ohm resistor (lower left) is for off board ground connections.
Wave Gen and a scope probe are connected to R2. Vout is the yellow wire with the other scope probe
on it.
You will measure the output of the Differential Amplifier for a few inputs to see how it works. First DC
inputs then some AC + DC inputs. You will need to use the 6V DC power supply (Yellow with Yellow
black connected to the other ‘GND’ connected between Blue and Green) set to the appropriate
voltage for one of the inputs otherwise the Wav Gen can supply the DC. Table 1 shows what the
inputs should be for the tests.
For the following measurements use the sine wave input as the reference for the ‘phase’ of the output
signal whether it is on Vin1 or Vin2. Some of the inputs have a DC offset as well.
Important Note: You will be calculating some gains. AC or DC gain can’t be calculated with a simple
equation if the corresponding input is 0 volts. This is true for the whole lab. i.e. 0 times any value of
gain is 0 and Vout/0 is ∞.
∆ VOUT
. Remember 0/0 = 1. So if Vin doesn’t
∆ VIN
change and Vout doesn’t change we are all set. However if Vout changes when Vin doesn’t change
then we can’t do much about calculating the gain.
To calculate DC gain when an input is 0 use the formula
When measuring AC signals make the V/Div as small as
possible so the displayed waveform is as close to full
screen as possible. (Green) Also put the GND in the
center. For DC signals put the GND reference near the
bottom of the screen and put the DC value close to the
top. (Blue) This makes the measured date when using
MEAS as accurate as it can be. The Yellow trace is too
small. Don’t use Auto Scale as it makes the traces small.
Vin1
DC
Vin2
AC
DC
Vout
AC
DC
AC
Phase
Case 1
0 VDC
0
0VDC
0
0
X
X
Case 2
1VDC
0
GND
GND
1V
X
X
Case 3
GND
GND
1VDC
0
-1V
X
X
Case 4
-1VDC
0
GND
GND
-1V
X
X
Case 5
GND
GND
-1VDC
0
1V
X
X
Case 6
-1VDC
0
1VDC
0
-2V
X
X
Case 7
0VDC
1VPP@1KHz
GND
GND
0
0.980Vpp
1.4
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Single Supply Op-Amp Amplifiers Procedure
Case 8
1VDC
1VPP@1KHz
GND
GND
1V
0.980Vpp
2.6
Case 9
-1VDC
1VPP@1KHz
GND
GND
-1V
0.980Vpp
-1.9
Case 10
GND
GND
0VDC
1VPP@1KHz
0
0.980Vpp
-179
Case 11
GND
GND
1VDC
1VPP@1KHz
-1V
0.980Vpp
180
Case 12
GND
GND
-1VDC
1VPP@1KHz
1V
0.980Vpp
178
Table 1: Differential Amplifier response table.
Please be aware that a few millivolts of change when the input signals are on the order of 1V doesn’t
represent any change in Vout for todays lab. It is just a small measurement error or op-amp offset.
Since this amplifier is only one circuit used over and over again the gain does not change between
different inputs. From the Theory Section what is the ‘gain’ of this differential amplifier (Read On)?
To determine this use the idea of superposition. Calculate the gain Vout/Vin1 with Vin2 grounded then
add the gain Vout/Vin2 with Vin1 grounded. Remember there are 2 inputs. Vin1 and Vin2. They
should show up in the gain equation. Remember the gain can be negative!
From the measured data: What is the actual gain for DC signals? Vout/Vin1___1___V/V, Vout/
Vin2___1__ V/V. and AC signals? Vout/Vin1___1___ V/V, Vout/Vin2___1__ V/V.
Are the AC and DC gains for each Vin the same.? ___Yes____ Should these gains be the same?
No, Vin2 is in inverting terminal while Vin1 is in non inverting so they should be same magnitude
different signs.
Determine the Algebraic, Numeric and measured gains. Note the algebraic gain will have R’s in the
equation. The numeric just numbers.
Algebraic Vout/Vin1 = __________R3/R1 (1-Vin2/Vin1) V/V _________. Numeric Vout/Vin1 =
____1 – Vin2/Vin1 V/V _____.
Measured Vout/Vin1 = ___1V/V___.
Algebraic Vout/Vin2 = _________R3/R1 (Vin1/Vin2 – 1) V/V __________. Numeric Vout/Vin2 =
___Vin/Vin2 – 1 V/V ______.
Measured Vout/Vin2 = __-1V/V____.
What is the output (Vout) of the circuit numericly? Vout = A1Vin1+A2Vin2 = 0V
IMPORTANT READ THIS!
We are going to simplify the differential amplifier by connecting the inverting input to ground. This sets
the differential gain to (Vin1 – 0) * 1. What comes out is supposed to be what goes in. This makes it
easier to see the various issues that come up when the op-amp circuit is converted from ±5V supplies
to a +5V supply. This is easy by connecting Vin2 to GND.
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Single Supply Op-Amp Amplifiers Procedure
Figure 2: Single ended non-inverting amplifier.
-5-
Single Supply Op-Amp Amplifiers Procedure
Use Table 2 to measure VIN1 and Vout for the different Vin configurations and calculate the gain.
∆ VOUT
When the input is 0 to calculate gain use the formula
. Remember that 0/0 = 1.
∆ VIN
VIN1 DC
VIN1 AC
VIN1
AC
Vout
DC
AC
Gain
DC
AC
DC
0VPP @ 1KHz
0VDC
0 and 0
0 and 0
Reference only
0VPP @ 1KHz
2.5VDC
0 and 2.5V
0 and 2.5V
X.
1
0VPP @ 1KHz
-2.5VDC
0 and -2.5V
0 and -2.5V
X.
1
3VPP @ 1KHz
0VDC
3.1Vpp and 0
3.06Vpp and 0
1
3VPP @ 1KHz
2.5VDC
3.1Vpp and
2.5V
3.06Vpp and 2.5V 1 1
3VPP @ 1KHz
-2.5VDC
3.1Vpp and
-2.5V
3.07Vpp and
-2.5V
X
11
Table 2: Single ended Non-inverting amplifier.
4. What is the AC gain? _____1_____
What is the DC gain? _____1_____
Are the gains the same.? ___Yes____
Should the gains be the same? ____Yes____
.
Most modern op-amp circuits have to work on single supplies. For example many mobile devices run
on a single battery voltage which gives V+ and ground. Let’s investigate this circuit for this type of
supply situation.
5. Build the single supply differential input amp shown in Figure 3.
• Note we are simply grounding the power supply pin 4 of the op-amp and eliminating the -5V
supply when compared to the last circuit.
Figure 3: Non-Inverting Amplifier with Single Supply.
Measure VIN1, VOUT, V2 and V3 with respect to GND. Remember on the scope for AC use Peak to
Peak and DC use Average Full Screen or preferably use the DVM (Digital Volt Meter) for DC. Be
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Single Supply Op-Amp Amplifiers Procedure
careful measuring V2 and V3. You may need to use a 10X probe setting to minimize injected noise to
the circuit.
IMPORTANT: If anything unusual (non-linear) is happening then gain is an invalid parameter and
shouldn’t be calculated. This is true for the whole lab. Also AC and DC voltages will be measurable
when non-linearity’s occur.
∆ VOUT
Remember to calculate DC gain use the formula
at DC.
∆ VIN
DC
Vout
AC
DC
Gain
AC
DC
AC
52 mV 0V
25mV 0V
X
X
10mV 0V
7.2mV 0V
Frequency
spike
3VPP@ 1KHz 0VDC
3.1V 0V
1.65V .5V
.53
X
800mV 0V
1.41V 0V
Yes
0V
0V 2.5V
0V 2.5V
11
0V 1.25V
0V 1.25V
No
3VPP@ 1KHz 2.5VDC
3.1V 2.5V
3.1V 2.5V
11
1.61V
1.25V
1.61V
1.25V
No
0V
0V -2.5V
00
11
0V 0V
0V -0.6V
No
3.1V -2.5V 0 0
00
0V 0V
135mV
-0.57mV
Yes
VIN1
AC
0V (Ref)
VIN1 DC
0VDC
2.5VDC
-2.5VDC
3VPP@ 1KHz -2.5VDC
AC
VIN1
V2
DC
AC
V3
DC
Unusual
Occurrences
Table 3:
If there are unusual occurrences capture the trace from the scope.
3Vpp at 1khz and 0V DC
It turns out that the sine wave driving the amplifier at ±XV with no offset presents a problem. Normally
this isn’t an issue if your amplifier can output positive and negative voltages. You removed the
negative supply so the amplifier can only put out positive voltages. The sine with -2.5VDC offset is
even worse than the 0V offset version of the input. Now we will try to remedy this problem.
Important Assumptions for the next question:
• Assume the op-amp is ideal.
• Assume the circuit is operating linearly. No clipping etc.
• Because it is linear you can use superposition to calculate the answer for DC separately from AC
if you want to.
• Because it is linear the voltage that appears at V3, the +input of the op-amp, is also forced to show
up at V2, the -input of the op-amp, by VOUT.
• To solve for VOUT/VIN: solve for V2 with respect to VOUT and V3 with respect to VIN. Then set V2 = V3
because they should be equal.
Describe VOUT / VIN1 with equations based on R1 to R4.
Vout /Vin1 = ((R1+R3)/R1) * (R4/(R4+R2)) = (20k/10k) * (10k/20k) = 1
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Single Supply Op-Amp Amplifiers Procedure
As was mentioned in the pre-lab we usually don’t have the luxury of applying a DC offset to the input
signal driving the amplifier. It is not practical! We usually have to supply a DC offset internal to the
amp design. One way is at the input stage of the op-amp. If this is done we also have to block the DC
coming through the inputs to the amp. We will do that now.
6. Build the circuit shown in Fig. 4.
• Vin2 is now set to a constant 2.5V. Use the 6V power supply set yo 2.5V for this.
• R1 and R4 are connected to 2.5VDC instead of GND (0VDC). Note both of these DC voltages are
AC grounds.
• A DC blocking cap, C2, is put in series with the input resistor R2.
Note the power supply connections are not shown any more. They still must be there for the amplifier
to work (Figure 3 shows them).
Figure 4: Non-Inverting Amplifier with Single Supply and DC offset on the input stage.
To calculate DC gain use the formula
VIN1
AC
VIN1 DC
AC
VIN1
DC
∆ VOUT
at DC
∆ VIN
Vout
AC
DC
Gain
AC
DC
AC
V2
DC
AC
V3
DC
Unusual
Occurrences
3VPP@1KHz 0VDC
3.1V 0V
3.2V 2.5V
1
X 1.61V 2.5V 1.61V 2.5V
No
3VPP@1KHz 2.5VDC
3.1V 2.5V
3.2V 2.5V
11
1.61V 2.5V 1.61V 2.5V
No
3VPP@1KHz -2.5VDC
3.1V -2.5V
3.2V 2.5V
1 -1
1.57V 2.5V
No
1.65V 2.5V
Table 3:
Capture any Unusual Occurrence traces.
Describe VOUT / VIN1 for AC signals with equations based on R1 to R4. Assume C2 is infinite.
Vout /Vin1 = ((R1+R3)/R1) * (R4/(R4+R2)) = (20k/10k) * (10k/20k) = 1
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Single Supply Op-Amp Amplifiers Procedure
One last change. Usually there isn’t a convenient voltage like the 2.5V source we used in the circuit
above on a portable device. It isn’t practical. You can however divide the +5V source down with a
resistor pair which must have a Thevenin equivalent R of the 10K resistor and a Thevenin source
voltage of 2.5V. The circuit shown in Figure 5 is just such a circuit. A proto-board example is shown
in the appendix with different 20K resistors than you have.
•
•
•
•
You might want to cut the leads on the resistors to make this circuit fit without shorting leads
together.
R6 and R7 are added. They are 20KΩ.
R1 and R4 are changed to 20KΩ.
R1 and R4 are connected to GND like in the original single ended input amplifier. Really connect
R1 to GND to remove some of the clutter on the proto-board.
Figure 5: Unity Gain Non-Inverting Amplifier with Single Supply and resistor generated offset.
•
Measure VIN1, VOUT, V2 and V3 all with respect to GND, AC and DC.
Remember to calculate DC gain use the formula
VIN1
AC
VIN1
DC
AC
VIN1
DC
∆ VOUT
at DC.
∆ VIN
Vout
AC
DC
Gain
AC
DC
AC
V2
1.6V 2.5V
1.6V 2.5V
No
DC
AC
V3
DC
Unusual
Occurrences
3VPP@1KHz
0VDC
3.2V 0V
3.2V 2.58V
1
3VPP@1KHz
2.5VDC
3.2V 2.5V
3.2V 2.5V
11
1.6V 2.5V
1.6V 2.5V
No
3VPP@1KHz
-2.5VDC
3.2V -2.5V
3.2V 2.5V
1 -1
1.6V 2.5V
1.6V 2.5V
No
X
Table 4:
Capture any Unusual Occurrence traces.
What is the AC gain VOUT/VIN1 of this circuit? _____1_______
What is the DC gain VOUT/VIN1 of this circuit? ______1 besides for -2.5V DC which is -1______
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Single Supply Op-Amp Amplifiers Procedure
Describe VOUT / VIN for AC signals with equations based on R1 to R7. Assume C2 is infinite for this
analysis.
V2 = V3
Vout/Vin = R3/ ((R7*R1)/(R7+R1)) = 10k / ((20k * 20k) / (20k+20k)) = 1
Appendix: A
Construction Example for Figure 5. With ±5V wires removed for clarity.
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