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ELCT1003: High Speed Electronic Circuits Current Feedback Operational Amplifier By: Ahmed Gamal Duha Yasser Eman Emad Nourhan El-Hamawy Reem Etman March 26, 2017 Outline Introduction Types of Feedback Amplifies Current Feedback Amplifiers Vs Voltage Feedback Amplifiers CFA circuit analysis Factors affecting CFA Circuits that employ CFA Using CFA in High speed electronics Introduction Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to feed back into itself. Introduction Why Feedback? There are several good reasons why feedback is applied and used in electronic circuits Circuit characteristics such as the systems gain and response can be precisely controlled. Circuit characteristics can be made independent of operating conditions such as supply voltages or temperature variations. Signal distortion due to the non-linear nature of the components used can be greatly reduced. The Frequency Response, Gain and Bandwidth of a circuit or system can be controlled to within tight limits. Introduction There are two types of feedback in amplifiers. They are positive feedback , also called regenerative feedback , and negative feedback , also called degenerative feedback. The difference between these two types is whether the feedback signal is in phase or out of phase with the input signal. Positive feedback occurs when the feedback signal is in phase with the input signal. A block diagram of an amplifier with positive feedback is shown in figure . Notice that the feedback signal is in phase with the input signal. This means that the feedback signal will add to or "regenerate" the input signal. Positive feedback increase overall gain. Introduction Negative feedback is accomplished by adding part of the output signal out of phase with the input signal. The methods of providing negative feedback are similar to those methods used to provide positive feedback. The phase relationship of the feedback signal and the input signal is the only difference. Negative feedback decrease overall gain. Negative feedback improves performance (gain stability, linearity, frequency response, step response) and reduces sensitivity to parameter variations due to manufacturing or environment. Types of Feedback Amplifiers I. Voltage-series: Output signal is voltage signal, feedback signal is voltage signal. Also called as seriesseries feedback. It is employed in voltage amplifiers. II. Voltage shunt: Output signal is voltage signal, feedback signal is current signal. Also called as series- shunt feedback. It is employed in Transresistance amplifiers. Types of Feedback Amplifiers III. Current shunt: Output signal is current signal, Feedback signal is current signal. Also called as shunt-shunt feedback. It is employed in current amplifiers. IV. Current series: Output signal is current signal, feedback signal is voltage signal. Also called as shunt-series feedback. It is employed in Transconductance amplifiers. Types of Feedback Amplifiers The operational transconductance amplifier (OTA) is an amplifier whose differential input voltage produces an output current. Thus, it is a voltage controlled current source (VCCS). The transconductance of the amplifier is usually controlled by an input current, denoted Iabc ("amplifier bias current"). Current Feedback Amplifier Vs Voltage Feedback Amplifier A VFA is conceptualized as a single stage in which the amplifier’s open-loop gain senses and amplifies a differential voltage. However, it’s generally realized as three stages: • Differential input stage to buffer and amplify the input signals, typically by a factor of five or 10. • A second stage to convert the differential signal to a singleended signal while providing very high gain (1000 to 10,000 V/V). • And a final stage, usually a low-output-impedance, unity-gain buffer that intermediates between the high-output impedance of the second stage and the load, while providing the current gain necessary to drive the load. For stability, the high-gain stage in the middle also includes a frequency-compensation capacitor. Open-loop gain is the product of the voltage gains of the three stages. Current Feedback Amplifier Vs Voltage Feedback Amplifier CFA consists simply of a unity-gain buffer with high input impedance between the non-inverting and inverting inputs. This is followed by an output stage whose voltage is equal to the current through the buffer, multiplied by transfer impedance, Z. CFAs are implemented as a high-ZIn unity-gain buffer between the non-inverting and inverting inputs followed by a transimpedance output stage. Current Feedback Amplifier Vs Voltage Feedback Amplifier Gain-Bandwidth Product • VFA has certain gain-bandwidth limitations. • Negative feedback sets fixed gain • The feedback impedance affects the VFA’s closed-loop gain and bandwidth. • The product of the two (called its gainbandwidth product) is constant, designers must trade off gain for bandwidth or vice versa. • CFA’s closed-loop gain is based upon the external components in the feedback loop. • CFA’s gain is essentially independent of frequency. • The CFA not only has higher bandwidth than the VFA, it also has an adjustable bandwidth. • CFA’s bandwidth is primarily a function of the values of the feedback resistor and the compensation capacitance. Current Feedback Amplifier Vs Voltage Feedback Amplifier Slew Rate How well the output can follow rapid changes in the input. • The amount of current available to charge and discharge the stabilizing capacitance limits slew rate. • • CFAs are inherently capable of much faster slew rates than VFAs. Essentially, the CFA’s slew rate is equal to the feedback current divided by the value of the compensation capacitor Slew-rate limitations are important because they affect total harmonic distortion (THD), which will limit the effective number of bits of a downstream analog-to-digital converter (ADC). Current Feedback Amplifiers Current Feedback Current Amplifier Transimpedance Amplifier Input: Current Output: Current Input: Current Output: Voltage Current Feedback Amplifier BJT Model Z Vin+ Vin- Vout Current Feedback Amplifier BJT Model • Input Stage: It consists of a unity gain buffer from Q1-Q4. The bias of this stage is generated by IB making Q2 and Q3 operate in their active region. • I to V Converter Stage: It consists of two modified Wilson current mirrors from Q5Q12. They sense the collector currents ic2 and ic3 and mirror them to node Z. Z Vin+ Vin- Vout Current Feedback Amplifier BJT Model • Compensation Capacitor: Due to the wide open loop BW of CFA, there is a risk of oscillations. It ensures that frequencies where oscillations might start are attenuated. The compensation capacitor prevents the charging current from saturating and thus the slew rate is very high. •Buffer Stage: It consists of transistors Q13Q18 which buffer the voltage at node Z to the output. It provides low output impedance. Z Vin+ Vin- Vout Cc Non-inverting CFA I Assume Z = Non-inverting CFA I Inverting CFA I Assume Z = Inverting CFA I Factors affecting Current Feedback Amplifier Stability Stability is independent of the input, and stability depends solely on the loop gain, Aβ. Breaking the loop at point X, inserting a test signal, VTI, and calculating the return signal VTO develops the stability equation. Factors affecting the design Ignoring the input and output buffer gain and output impedance for simplicity, we obtain these three equations: Therefore the stability equation is given by: Factors affecting the design For both inverting and non-inverting circuits the stability equation remains the same. This comes as no surprise because the two op amp parameters that affect the stability are the transimpedance Z and the input buffer’s output impedance, ZB. When ZB = 0 Ω and ZF = RF the loop gain equation is; Aβ = Z/RF. Under these conditions Z and RF determine stability, and a value of RF can always be found to stabilize the circuit. The equation id found to be : When ZB approaches zero, the closed-loop gain term also approaches zero, and the op amp becomes independent of the ideal closed-loop gain. Under these conditions RF determines stability, and the bandwidth is independent of the closed-loop gain. Factors affecting the design Effect of parasitic capacitance at the inverting input node: The parasitic capacitance is a result of the board layout on the inverting input node to ground causing the impedance ZG to become reactive. Factors affecting the design The graph illustrates the effects of stray capacitance on CFA closed-loop performance. Factors affecting the design Feedback Capacitance The first equation describes the feedback impedance, when stray capacitance is formed across the feedback resistor. The second equation gives the loop gain with the new impedance values. This loop gain transfer function contains a pole and zero, thus, depending on the pole/zero placement, oscillation can result. Compensation of input and feedback capacitances When both CG and CF are present, they can be adjusted so as to cancel each other out. The stability equation is then described by Setting the pole equal to the zero yields Since RB is the dominant resistance, the given equation can be reduced to Limitations of the Current Feedback Amplifier • The inverting node • Stability vs. bandwidth: inversely related • Lack of precision: but most high frequency applications do not need precision • Unity gain circuit needs extra components than VFA • Application difficulty Circuits that employ CFA The term current feedback refers to the internal operation of the op-amp, not some new and exotic way of connecting the output back to the input. Circuits that employ CFA : • Gain Circuits with inverting and non inverting configurations. • Different topologies of filter circuits. Circuits that employ CFA Gain Circuits There are two gain configurations inverting and non inverting. Designers may notice that the circuit topologies are identical to those of voltage feedback amplifiers. While this is true, there are some minor peculiarities: • Designer has to follow the recommendation of Rf given in the datasheet as it is considered as an important factor that affects the circuit stability . • The inverting configuration has extremely low input impedance, and therefore is not very useful. Circuits that employ CFA Filter Circuits Filter circuits are used to remove unwanted harmonic components from signals, while leaving the components that are of interest. Single Pole Filter Single pole filters have a roll off of 20 dB per decade in the stop band. They come in low-pass and high-pass varieties. The varieties are all non inverting, and have the option of unity gain or gain set by the feedback resistor Rf and R2. Circuits that employ CFA Filter Circuits Twin T Filters The Twin T filter topology is the only multiple op-amp topology suitable for current feedback op-amps. Rf must be used instead of a short from output to inverting input of both op-amps. The advantage of the Twin T topology is that it is easier to tune than the Sallen-Key. Of course the penalty is the addition of a second op-amp and some passive components. Circuits that employ CFA Filter Circuits Sallen-Key Filters The non inverting Sallen-Key topology is well suited to current feedback op-amps. The filter can be operated in the unity gain mode or can be operated with a gain. The resistance that exists from output to the inverting input is to modify the gain and the bandwidth while using the current feedback amplifier. Circuits that employ CFA Filter Circuits Sallen-Key Filters Circuits that employ CFA Filter Circuits Sallen-Key Filters Circuits that employ CFA Filter Circuits Sallen-Key Filters Circuits that employ CFA Filter Circuits Sallen-Key Filters Circuits that employ CFA Filter Circuits Sallen-Key Filters Circuits that employ CFA Filter Circuits Sallen-Key Filters Circuits that employ CFA Filter Circuits Sallen-Key Filters Using CFA in high speed electronics • Current-feedback op-amp architecture has emerged to become a dominant solution for many applications. Possessing a number of strengths, this amplifier architecture can be used in nearly any application that calls for an op amp, because it does not have a constant gain-bandwidth product. the bandwidth of a CFB op amp is proportional to the feedback resistor . • For every CFB op amp there is a recommended value of feedback resistor . If you increase the value of the resistor beyond this value, you reduce the bandwidth, and if you decrease the value of the resistor , you increase the bandwidth. • significantly increased the designer’s ability to solve difficult high speed amplifier problems. The current feedback architecture has very high slew rate Using CFB in high speed electronics • This figure shows what happens to the bandwidth as you change the feedback resistor. At the far right-hand curve where the RF = 147Ω, the frequency response has peaked quite substantially. • This curve also has the highest bandwidth. Decreasing the resistor too far below 147Ω results in ringing on your pulse response, and it will actually oscillate. Using CFB in high speed electronics • Next figure is from a datasheet of one of the CFA (AD844) device and it shows the suggested feedback resistor for each gain. • the recommendation for a gain of two is the 300Ω resistor, with the best combination of gain flatness, settling time and speed. Applications 1) Reducing system noise. • Minimizing noise is particularly important if you are building an IF amplifier or a low-frequency RF amplifier, With current-feedback amplifiers increasing the feedback resistor can often reduce system noise. This is because the frequency response falls off faster than the resistor noise goes up. • To reduce noise to the portions of the circuit following the amplifier, it is very important to have only the bandwidth necessary and no more. Besides using the best value of feedback resistor, you can add additional filtering to the circuit Applications 2) Video distribution amplifier. 3) Video line drivers. 4) Medical Imaging. 5) RGB video driver. 6) broadcast video systems. 7) high resolution projectors. 8) ADC Drivers. and Radar Systems. Example Comparing Voltage and Current Feedback Op Amps • VFB amplifiers offer: • Lower Noise • Better DC Performance • Feedback Freedom • CFB amplifiers also tend to offer: • Faster Slew Rates • Lower Distortion • Feedback Restrictions Formula Sheet General equation of stability: Parasitic capacitance at V- : Parasitic capacitance at the feedback : Stability equation with parasitic capacitances : Cancelling the effect of both capacitances: References 1. http://electronicdesign.com/analog/what-s-difference-between-voltage-feedback-andcurrent-feedback-op-amps 2. http://www.analog.com/en/analog-dialogue/articles/current-feedback-amplifiers-2.html 3. http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=sloa066&file Type=pdf 4. http://www.intersil.com/content/dam/Intersil/documents/an94/an9420.pdf 5. http://www.intersil.com/content/dam/Intersil/documents/an94/an9420.pdf 6. http://www.embedded.com/design/other/4006793/Back-to-the-basics-Using-currentfeedback-op-amps-for-high-speed-designs 7. http://www.ti.com/lit/an/sloa066/sloa066.pdf 8. http://www.analog.com/media/en/training-seminars/tutorials/MT-034.pdf