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Transcript
Current Feedback Op-Amp
BY MAHMOUD EL-SHAFIE
Lecture Contents
• Introduction
• Operation
• Applications in High Speed Electronics
Historical Background
Vacuum tube
op-amp (1953)
solid-state, discrete
op-amp (1961).
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
Ref 1
Open-Loop Amplifier
Closed Loop Amplifier
2. Classification of Amplifiers
Types
Voltage Amplifiers
Input
Voltage
Output
Voltage
Current Amplifiers
Current
Current
Transconductance Amplifiers
Voltage
Current
Transresistance Amplifiers
Current
Voltage
Question
what is the Difference between voltage feedback and Current
Op-Amps ?
Voltage Feedback Amplifier :
The non inverting gain configuration amplifies the difference voltage,
(VIN+ - VIN-), 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 RF and
RG.
Voltage Feedback Amplifier , Ref 2
To derive the closed-loop transfer function of this circuit, Vo/VIN+, 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
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.
Current Feedback Amplifier , Ref 2
Z(s) is the high impedance node
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 VIN- node, while the buffer maintains VIN+ = VIN-.
If we assume, for the moment, that the buffer has zero output
resistance, then Ro = 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 RF,
Current Feedback Amplifier , Ref 2
Thus, the closed-loop bandwidth of a current feedback amplifier will vary
with the value of RF, but not with the noise gain 1 + RF/RG
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 .
Why shall I Choose CFAs over VFAs ?
* 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
VERY IMPORTANT
SLEW RATE
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.
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
Ref.5
Example :
Example :
Ref. 6
REFERENCES
1. http://eee.guc.edu.eg/CorsMain/Electronics/ELCT604%20Electronic%20Circuits/schedule.html
2. http://www.analog.com/library/analogDialogue/Anniversary/22.html
3. http://www.ieee.li/pdf/viewgraphs/current_feedback_vs_voltage_feedback_amplifiers.pdf
4. http://electronicdesign.com/analog/what-s-difference-between-voltage-feedback-and-current-feedback-op-amps
5. http://www.analog.com/media/en/training-seminars/tutorials/MT-057.pdf
6. http://eee.guc.edu.eg/Courses/Electronics/ELCT604%20Electronic%20Circuits/sheets/4.Sheet-FB%20amplifiers.pdf