Department of Electrical and Computer Engineering

Current Feedback Operational Amplifier

Jon Lepp

ECE 312 Analog Electronics

Final Project

12/11/2015

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Objective

The objective of the final project is to design a current feedback operational amplifier (CFOA). In the design chosen for this project, the Wilson current mirror is chosen over the standard current mirror making a total of 24 transistors in the final design to improve the amplifier performance. The simulations included in the final project include a DC analysis to find the operating points in the circuit, the transient response of the circuit to test for a closed loop gain of 2 and calculate the slew rate, as well as a bode and phase plot to find the bandwidth and poles of the current feedback operational amplifier.

Schematic and Simulations

The schematic in Fig P.1 shows the schematic of the final design used for the transient simulation, the bode plot, and the phase plot. This schematic includes the Vin input voltage and the feedback loop which is connected to the output. In the schematic there are four Wilson current mirrors. All of the resistors in the circuit are chosen to be the same value so the current is mirrored through each of the stages of the current feedback operational amplifier.

Fig P.1 (Transient and Bode Plot Circuit)

Fig P.2 shows the transient simulation for the circuit in Fig P.1. Using subplots in the simulation window, the input is plotted on the first plot. The amplitude is simulated at 200 mV. For the given circuit in Fig P.1, the output is designed to have a gain of two using the feedback loop. The amplitude of the output is shown in the second subplot of Fig P.2. As shown the amplitude of the output is double the amplitude of the input, thus verifying the amplifier’s gain

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of two. The last subplot shows the input and the output on the same plot so they can be easily compared.

Fig P.2 (Transient Simulation)

Fig P.3 shows the circuit used to find the DC operating points of the circuit. In this schematic, the small signal AC voltage is removed from the circuit. The feedback loop is also removed from the circuit. The input and output terminals of the circuit are grounded to create a closed circuit. Most of the DC operating points are displayed at their various points throughout the circuit. Upon careful inspection of the voltages throughout the circuit, there are various points in the circuit where the voltages are the same. This is expected because the resistors and transistors are the same throughout the circuit and the current mirrors are replicating the current throughout the various stages of the amplifier.

Fig P.3 (DC Operating Point Circuit)

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Fig P.4 shows all of the DC operating points throughout the circuit. As expected there are several repeated values for the voltages at various points throughout the circuit. For example V(n006), V(n002), V(n005), V(n004), V(noo3), V(n001) all have similar voltages of 2.9 volts at their node locations in the circuit. There are also several values of -2.38 volts that are repeated in the table as well as -3.26 volts. All of the voltages are at the node locations within the current mirrors of the circuit. This is why they are repeated multiple times. This is expected.

Fig P.4 (DC Operating Point Voltages)

Fig P.5 shows the first simulated bode and phase plot for the circuit in Fig P.1. The stop frequency for this bode plot is simulated out to a frequency of 100 megahertz. Upon further inspection of the bode plot, the amplifier appears to be acting as a low pass filter with a cutoff frequency around 10 megahertz. For experimental purposes, another bode plot was simulated to get a broader frequency spectrum to see how the circuit truly behaves at even higher frequencies. This bode plot is shown in Fig P.6. It to, appears to have the characteristics of a low pass filter with a cutoff frequency of 10 megahertz. It also shows the frequency and phase behavior all the way out to frequencies of 100 gigahertz. The oscilloscopes used in the experimental procedure have a maximum frequency of 20 megahertz. This means the bode plot created in the experimental procedure for this lab will not show all of what happens at very high frequencies such as 100 gigahertz. However, from the simulations shown in Fig P.5 and P.6, we can expect to see a cutoff around 10 megahertz.

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Fig P.5 (Bode and Phase Plot)

Fig P.6 (Extended Bode and Phase Plot)

The final simulation for this lab is a square wave input into the current feedback operational amplifier. The input is given as two volts peak to peak for the square wave pulse. The output shown in Fig P.7 does not have a gain of two for a square wave input for the circuit.

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Fig P.7 (DC waveform input)

Data

The following pictures show the input stages of the CFOA with their corresponding DC voltages at different nodes in the circuit. To prove the circuit is working correctly, measure a few different spots in the circuit and compare some of the different voltages from the measured values and the simulation. As shown, the CFOA appears to be working as expected and simulated because the DC voltages appear to be close in value and have the same signs for different values and areas in the circuit.

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The following data tables show the frequency profile for the CFOA. Table T.1 contains the frequency set by the function generator, the V ¿ voltage, which is assumed to remain constant at 200 mV throughout the data collection process, the V out voltage corresponding to the frequency where the measurement is taken at the output, and the gain calculation. In Table T.1, the gain is simply taking V out /V ¿. This gain is not measured in decibels yet. Table T.2 takes the gain from

Table T.1 and multiplies the value by 20∗log(V outV ¿) to get the gain in the dB scale. Table T.2

also contains the frequencies where each corresponding gain values can be found. Table T.2 is used to make the bode plot of the CFOA shown in Fig P.8. The bode plot in Fig P.8 has a large spike in gain just prior to reaching 10 MHz. For frequencies below 1 MHz, the gain remains constant for the CFOA. For all frequencies after 10 MHz, the gain of the amplifier shows a rapid reduction in gain as the frequencies get higher.

Fig P.8 (Bode Plot of CFOA)

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Table T.1 Table T.2

Frequency (kHz) Vout (V) Vin (V) Gain (Vout/Vin)1.000 0.393 0.200 1.9655.000 0.393 0.200 1.965

10.000 0.395 0.200 1.97520.000 0.395 0.200 1.97540.000 0.397 0.200 1.98580.000 0.401 0.200 2.005100.000 0.401 0.200 2.005200.000 0.398 0.200 1.990400.000 0.406 0.200 2.030800.000 0.410 0.200 2.050

1000.000 0.408 0.200 2.0402000.000 0.442 0.200 2.2104000.000 0.586 0.200 2.9328000.000 0.886 0.200 4.4308500.000 1.439 0.200 7.1959000.000 1.793 0.200 8.9659100.000 1.817 0.200 9.0859200.000 1.809 0.200 9.0459300.000 1.656 0.200 8.2809400.000 1.504 0.200 7.5209500.000 1.367 0.200 6.8359600.000 1.282 0.200 6.4109700.000 1.170 0.200 5.8509800.000 1.116 0.200 5.5809900.000 1.073 0.200 5.36510000.000 1.003 0.200 5.01511000.000 0.787 0.200 3.93512000.000 0.578 0.200 2.89013000.000 0.484 0.200 2.42014000.000 0.393 0.200 1.96515000.000 0.340 0.200 1.70016000.000 0.342 0.200 1.71017000.000 0.356 0.200 1.78018000.000 0.349 0.200 1.74519000.000 0.332 0.200 1.66020000.000 0.354 0.200 1.770

Gain (dB) Frequency (kHz)5.867 1.0005.867 5.0005.911 10.0005.911 20.0005.955 40.0006.042 80.0006.042 100.0005.977 200.0006.150 400.0006.235 800.0006.193 1000.0006.888 2000.0009.343 4000.00012.928 8000.00017.141 8500.00019.051 9000.00019.166 9100.00019.128 9200.00018.361 9300.00017.524 9400.00016.695 9500.00016.137 9600.00015.343 9700.00014.933 9800.00014.591 9900.00014.005 10000.00011.899 11000.0009.218 12000.0007.676 13000.0005.867 14000.0004.609 15000.0004.660 16000.0005.008 17000.0004.836 18000.0004.402 19000.0004.959 20000.000

The following oscilloscope captures show several pieces of information for the current feedback operational amplifier. The oscilloscope capture OC.1 shows a low frequency transient response for the CFOA. The simulated input voltage was taken to be 200 mV. This oscilloscope capture

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was taken at a frequency of 4 kHz. As shown by the amplitudes in the figure on the right side of the oscilloscope, the amplifier still has a gain of two at a frequency of 4 kHz.

The oscilloscope capture in OC.2 displays what starts to happen to the signal as the frequency becomes higher than 10 MHz. The signal starts looking worse and worse the higher up in frequency the input becomes. This verifies, once again that the cutoff frequency is at 10 MHz for the CFOA circuit.

OC.1 OC.2

The oscilloscope capture OC.3 shows the output of the CFOA when a DC pulse is used as an input signal to the CFOA at a low frequency of 4 kHz. The output has a unique shape at the bottom of the wave and is not perfectly square as you would expect it to be. If you zoom in on the waveform and increase the frequency, as was done in the oscilloscope capture OC.4, you begin to see the noise in the circuit. Despite this small amount of noise in the signal, the CFOA is still getting a gain of two even for the square waveform. This differs from the simulated results where the gain remained less than two.

OC.3 OC.4

For experimental purposes, the last described step of the lab was repeated twice more to

see what the output from a square wave would look like at extremely high frequencies. As shown, the output wave becomes more and more distorted as the frequency gets higher and higher as shown in the oscilloscopte captures OC.5 and OC.6.

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OC.5 OC.6

The following oscilloscope captures are used to find the slew rate of the CFOA. The

input to the CFOA is still a square wave pulse, however the period is changed to zoom in on one particular rising edge of the pulse. The slew rate is the slope of the wave shown given in volts per microsecond. The oscilloscope capture OC.7 will be used to calculate the slew rate for the CFOA because the x cursors were placed correctly for the calculation. OC.8 could be used to calculate the slew rate but it wouldn’t be very accurate because the right x cursor is placed at the center of the highest peak. This oscilloscope capture was taken to see the difference in slew rates based on these two positions.

Slewrate= ΔyΔx

= 2.94V0.222µ s

=13.27V /µs

The slew rate is used to determine how fast the amplifier is. The ideal slew rate for the CFOA is 30 volts per microsecond. For this design, the CFOA is operating at just under half of the ideal speed of the CFOA.

OC.7 OC.8

The following picture in Fig P.9 shows the CFOA design implemented as a circuit. This

is the same circuit as Fig P.1. The design was kept neat with mostly small wires used with the exception of the outputs that had to be brought to a different area of the breadboard for lack of space. This circuit was relatively easy to built. The hardest part was figuring out which direction the transistors had to be facing to operate correctly.

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Fig P.9 (CFOA Circuit)

Additional Research on CFOA

The CFOA is faster than the voltage feedback amplifier. It has several advantages over the voltage feedback amplifier such as a high slew rate and larger bandwidth. They are able to account for higher frequencies of sinusoidal, triangular, and square waves. They also implement less external components (resistors, etc.) resulting in an overall better design. Some disadvantages of the voltage feedback operational amplifier include their input offset voltage, inputs offset current, common mode input range, common mode rejection ratio (CMRR), power supply rejection ratio (PSRR), their open loop gain, and the characteristic that they tend to be noisier than VFOA. For simple linear applications, the CFOA and VFOA can be used

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interchangeably. However, in nonlinear applications such as an integrator circuit, they may not be able to be used interchangeably. The CFOA operates faster because it operates in current mode making it less prone to stray node capacitances. The CFOA should not be used in high gain applications particularly when absolute gain accuracy is required. These are a few of the characteristics, advantages, and disadvantages of the CFOA.

Conclusion

The purpose of this experiment was to design a current feedback operational amplifier with a gain of two shown in the transient analysis of the circuit. The circuit was also analyzed to find the DC operating points and the frequency response of the circuit to create a bode plot to analyze the CFOA’s characteristic of having a high bandwidth. In addition, the slew rate of the CFOA was found to analyze the speed of the circuit. Finally, additional research was conducted on the CFOA to find other common characteristics including advantages and disadvantages of the CFOA.