Download Lecture_current feedback amplifier

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