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This version (04 Jan 2021 00:31) was approved by Doug Mercer.The Previously approved version (03 Jan 2021 22:12) is available.Diff

Activity: Making a full Operational Amplifier from previous blocks

Objective:

By combining some of the circuit blocks already explored, the goal of this lab activity is to build a complete three stage operational amplifier from a few discrete devices. The differential amplifier used as the first stage is combined with a Miller compensated common emitter second stage and a class A-B push-pull output third stage.

Notes:

As in all the ALM labs we use the following terminology when referring to the connections to the ALM1000 connector and configuring the hardware. The green shaded rectangles indicate connections to the ALM1000 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.

Three stage operational amplifier

Materials:

ADALM1000 hardware module
Solder-less breadboard and Jumper wire kit
12 – Resistors, various values
2 – Capacitors
1 – 1N914 or similar diode
5 – 2N3904 NPN transistors, for Q1 and Q2 Q3, Q4 and Q5
2 – 2N3906 PNP transistors for Q6 and Q7
1 – TIP31 NPN power transistor Q8 (or 2N3904)
1 – TIP32 PNP power transistor Q9 (or 2N3906)

Directions:

The object here is to construct the circuit shown in figure 1 using only the various transistors available in the ADALP2000 Analog Parts Kit. You should build the circuit on your solder-less breadboard. The values of the input and feedback resistors are shown as 4.7KΩ but could be changed to produce other gains. The amplifier is shown in a non-inverting configuration but inverting configurations are also possible.

Figure 1, Three Stage Operational Amplifier

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 2 and a photo of the board is shown in figure 3.

Experiment board design files

Figure 2, Operational Amplifier PCB schematic.

The PCB uses the standard 8 pin DIP single op-amp footprint and can be inserted into a solder-less breadboard.

Figure 3, Operational Amplifier PC Board.

Hardware Setup:

Connect your circuit to the ALM1000 analog I/O connector as indicated by the green boxes.

Procedure:

Configure the channel A generator output CA-V for a 500 Hz sine wave with Min value of 1.5 V and Max value of 3.5 V. Using scope channel CB-H to observe the output of the amplifier, record the input to output amplitude and phase relationship.

Lower the Min value until the amplifier output clips on the negative peaks of the sine wave. Record this value and explain. Increase the Max value until the amplifier output clips on the positive peaks of the sine wave. Record this value and explain.

Frequency compensation

C1 provides frequency compensation in order to reduce the gain at high frequencies where it may be subject to oscillation (instability). It also sets the amplifier slew-rate. Change the shape of channel A to square wave. Set the frequency to 2000 Hz. Change the value of C1 to 4.7nF (marked 472) and measure the positive and negative slew rate of the amplifier. The DC current supplied by Q5 determines how fast the compensation capacitor C1 can be charged. Using the calculated current in Q5 and the value of C1 estimate what the slew rate should be and compare this with what you measured.

Shortcomings

While there are no perfect op amps, some monolithic devices are very, very good. This circuit has some real shortcomings as follows:

No provision for thermal stability – operate at room temperature only.
Relatively high input offset voltage, but can be nulled.
Lack of output overcurrent protection.
Limited open loop voltage gain – the open loop voltage gain of a monolithic device is at least an order of magnitude higher.
Potentially high crossover distortion

Questions:

Measure quiescent power supply current.

Null input offset voltage – set up with a voltage gain of 100, ground input and adjust R2 or R3 so that the output voltage is zero

Observe thermal stability – after the input offset voltage has been nulled, blow hot air on different devices in the circuit and watch the output voltage shift (carefully use a hair dryer)

Measure input bias current – add high value 100K resistor in series with input terminals, measure voltage drop across resistor, then calculate the current.

Plot frequency response at 0.5 Vp-p output and at full output voltage (use large value for C1)

Measure open loop gain – set up as an inverting amplifier – with full output voltage, measure input node voltage – divide AC output voltage by AC input node voltage (note, this cannot be done with a monolithic op amp due to its extremely high open loop gain).

Determine maximum slew rate in both polarities (positive going and negative going)

Perhaps you can experiment upon and improve upon this relatively simple circuit design

For Further Reading:

Increase amplifier output drive using a push-pull stage

Return to Lab Activity Table of Contents

university/courses/alm1k/alm-lab-13.txt · Last modified: 04 Jan 2021 00:31 by Doug Mercer