By combining the circuit blocks already explored, the goal is to build a complete high open loop gain amplifier from a few discrete devices.
ADALM2000 Active Learning Module
1 - 8.2KΩ Resistor (close approx. can be made by connecting your 1.5KΩ and 6.8KΩ in series)
1 - 47KΩ Resistor
1 - 100KΩ Resistor
2 - 470KΩ Resistor
1 - 10KΩ Resistor
1 - 1KΩ Resistor
2 - 22uF capacitor
1 - 1uF capacitor
1 - 47nF capacitor
1 - Small signal PNP transistors (2N3906)
3 - Small signal NPN transistors (2N3904 SSM2212)
On your solder-less breadboard construct the amplifier circuit shown in figure 1 below. The breadboard connections are shown in figure 2. The green boxes indicate connections to the connector on ADALM2000.
Figure 1 Three stage amplifier
Connect your circuit to the ADALM2000 I/O connector as indicated by the green boxes. It is best to ground the unused negative scope inputs when not being used. If the SSM2212 NPN matched pair is used then it is best to use it for Q1 and Q2.
Figure 2 Three stage amplifier Breadboard Circuit
Configure waveform generator for a 1 KHz sine wave with an amplitude of 400 mV peak-to-peak and zero offset. Using scope channel 1 to observe the input at W1 and scope channel 2 to observe the output of the amplifier at RL, record the input to output amplitude and phase relationship.
Figure 3 Three stage amplifier Waveforms
What is the DC voltage seen at the base of Q1? What sets this DC level?
What is the gain from the input source, W1, to the output seen at RL? Which components set this gain and why?
Run a computer simulation of the amplifier and calculate the open loop gain as seen from the base of transistor Q1 to the output at the collector of Q4. Report this gain vs. frequency.
Change the value of the load resistor RL. How does lowering the value of RL affect the open loop and closed loop gain and why?
Change the value of compensation capacitor C3. How does raising and lowering the value of C3affect the frequency response and why?
What happens if C3 is completely removed and why?
What happens when C2is removed and why?
By combining some of the circuit blocks already explored, the goal is to build a complete unity gain buffer amplifier. The addition of the current mirror load for the differential stage is a key improvement to this simple amplifier.
ADALM2000 Active Learning Module
1 - 15KΩ Resistor (a 10KΩ in series with a 4.7KΩ can be substituted)
2 - Small signal PNP transistors (2N3906, or SSM2220 PNP match pair can be used)
6 - Small signal NPN transistors (2N3904, use SSM2212 NPN matched pair for Q1 and Q2 A TIP31C may be substituted for Q5 if you don't have enough 2N3904 devices)
Construct the circuit shown in figure 4 on your solder-less breadboard. The breadboard connections are shown in figure 5.
Figure 4 Amplifier with unity gain
Connect your circuit to the ADALM2000 I/O connector as indicated by the green boxes. It is best to ground the unused negative scope inputs when not being used.
Figure 5 Amplifier with unity gain Breadboard Circuit
Configure AWG1 for a 1 KHz sine wave with an amplitude of 2 V peak-to-peak and zero offset. Using scope channel 1 to observe the input at W1 and scope channel 2 to observe the output of the amplifier, record the input to output amplitude and phase relationship.
Figure 6 Amplifier with unity gain Waveforms
Here is a good technical paper on how to make Simple Op Amp Measurements.
Return to Lab Activity Table of Contents
PC board design files for this experiment, and other related extensions, can be found on the ADI GitHub education tool repository. The PCB schematic is shown in figure 3 and a photo of the board is shown in figure 4. Component placement is shown in figure 5.
Power and bias rail decoupling capacitors C2, C3 and C4 are optional. Pin sockets are best used for Frequency compensation capacitor C1 to allow for experimenting with different values.
Resistor R4 sets the bias current for the first and second stages based on the power supply voltage. The value can be adjusted based on the range of supply voltages the amplifier will be operating. For +5 operation 1.5kΩ is a good working value. For 10 V (+/- 5V) a 3.3kΩ is a good working value.
Resistors R5 and R6 set the steady state bias current in the output stage. Using 2N3904 and 2N3906 in the output stage, R5 = 6.8kΩ and R6 = 10kΩ is a good safe starting point.
Output emitter resistors R7 and R8 can be any small value in the range of 2.7 to 10 ohms.
Figure 3, Operational Amplifier PCB schematic.
The PC Board version with the standard 8 pin DIP single op-amp footprint is shown in figure 4. A version with all the pins in a single row (SIP) footprint is shown in figure 5. Either version can be inserted into a solder-less breadboard.
Figure 4, Example constructed Operational Amplifier PC Board, DIP version.
Figure 5, Example constructed Operational Amplifier PC Board, SIP version.
To make it somewhat easier to install the components, figure 6 for the DIP version and figure 7 for the SIP version are provided.
Figure 6, DIP PC Board component placement.
Figure 7, SIP PC Board component placement.