Current Feedback Op-Amp BY MAHMOUD EL-SHAFIE

Lecture Contents Introduction Operation Applications in High Speed Electronics

Historical Background solid-state, discrete op-amp (1961). Vacuum tube op-amp (1953) high speed hybrid IC op-amp (1979)

1.Introduction Feedback : Is the process where by a portion of the output is returned to the input to form a part of the system excitation Open-Loop Amplifier Closed Loop Amplifier Ref 1

2. Classification of Amplifiers TypesInputOutput Voltage AmplifiersVoltage Current AmplifiersCurrent Transconductance AmplifiersVoltageCurrent Transresistance AmplifiersCurrentVoltage

Question what is the Difference between voltage feedback and Current Op-Amps ?

Voltage Feedback Amplifier : The non inverting gain configuration amplifies the difference voltage, (V IN + - V IN -), by the open loop gain A(s) and feeds a portion of the output back to the inverting input through the voltage divider consisting of R F and R G. Voltage Feedback Amplifier, Ref 2

To derive the closed-loop transfer function of this circuit, V o /V IN+, assume that no current flows into the op amp (infinite input impedance) both inputs will be at about the same potential with Voltage Feedback Amplifier, Ref 2

Current Feedback Amplifier - The non inverting input is the high-impedance input of a unity gain buffer, and the inverting input is its low-impedance output terminal. - The error current is mirrored to a high impedance node, where it is converted to a voltage and buffered at the output. Z(s) is the high impedance node Current Feedback Amplifier, Ref 2

The high-impedance node is a frequency-dependent impedance, it has a high dc value and rolls off at 20 dB/decade Current Feedback Amplifier, Ref 2

The closed-loop transfer function is found by summing the currents at the V IN- node, while the buffer maintains V IN+ = V IN-. If we assume, for the moment, that the buffer has zero output resistance, then R o = 0 ohms Current Feedback Amplifier, Ref 2

The closed-loop transfer function for the current feedback amplifier is the same as for the voltage feedback amplifier, but the loop gain (1/LG) expression now depends only on R F, Current Feedback Amplifier, Ref 2

Thus, the closed-loop bandwidth of a current feedback amplifier will vary with the value of R F, but not with the noise gain 1 + R F /R G Current Feedback Amplifier, Ref 2

Loop Gain is Everything in Op Amps Op Amp suppliers are essentially selling a device that does impedance transformation (high input Z to low output Z ) and a whole lot of open loop gain. The customer then closes the loop to get a more controlled voltage gain, but also gets a huge improvement in precision (both DC and AC) due to the high open loop gain. For high frequency parts, the DC open loop gain is a secondary issue where the magnitude of the open loop gain equals the inverse of the feedback ratio. While the closed loop response is what is normally observed and reported, where loop gain over frequency is used to check for distortion and stability analysis.

Operational amplifiers are analog circuits that amplify voltage based on voltage or current difference across their differential inputs. * VFAs : voltage-feedback amplifiers CFAs : current-feedback amplifiers Why shall I Choose CFAs over VFAs ?

1- Essentially unlimited slew rate - gives very high full power bandwidth 2- Gain bandwidth independent Most useful aspect of this is intrinsic low gain stability with very high closed loop band width. 3- Most CFB also provide a large output current drive capability 4- Applications such as adder and high gain applications are ideal target applications Ref 3

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). The CFA not only has higher bandwidth than the VFA, it also has an adjustable bandwidth. Instead of a constant gain-bandwidth product, the CFA’s bandwidth is primarily a function of the values of the feedback resistor and the compensation capacitance. SLEW RATE VERY IMPORTANT Ref 4

Classical Tradeoff’s in selecting Current Feedback Op Amps – Although input voltage noise can be low, inverting input current noise is always much higher than VFB equivalents This limits the usability of the CFB for receivers. Most time a VFB will offer better dynamic range. – Feedback element is constrained in its impedance range since it is the compensation element This limits the usability of the CFB for transimpedance. A CFB can be used in transimpedance applications but mostly to low gain as the feedback resistor is the compensation element. Too little and you have oscillation, to much and the bandwidth becomes quickly limited.

Summary Most VFB devices are low gain stable and can give the lowest noise and distortion at low gains and frequencies. Non-inverting differential I/O stages work pretty good here. For moderate performance targets, all CFB devices are low gain stable and do well to very high output powers.

Summary Most VFB devices are low gain stable and can give the lowest noise and distortion at low gains and frequencies. Non-inverting differential I/O stages work pretty good here. For moderate performance targets, all CFB devices are low gain stable and do well to very high output powers. CFB devices at higher gains, and particularly inverting, are probably lower noise and can deliver a lower distortion to higher gains. Inverting differential I/O are the best for HD2 suppression.

CFB OP AMP SIMPLIFIED CIRCUIT AND MODEL -We will now examine in more detail the current feedback (CFB) opamp topology which is very popular in high speed op amps. -As mentioned previously, the circuit concepts were introduced decades ago, however modern high speed complementary bipolar processes are required to take full advantage of the architecture.

It has long been known that in bipolar transistor circuits, currents can be switched faster than voltages, other things being equal. This forms the basis of non-saturating emitter-coupled logic (ECL) and devices such as current-output DACs. The current mirror is a good example of how currents can be switched with a minimum amount of delay

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Example :

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