Current Voltage Feedback Amplifiers Debbie Brandenburg Janua

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Current Voltage Feedback Amplifiers Debbie Brandenburg Janua

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Current Voltage Feedback Amplifiers
Debbie Brandenburg
January 1998
question continuously troubles analog design engineer: "Which amplifier topology better application, current feedback voltage feedback?" most applications, differences between current feedback (CFB) voltage feedback (VFB) apparent. Today's amplifiers have comparable performance, there certain unique advantages associated with each topology. general, amplifiers offer:
Gain
Lower Noise Better Performance Feedback Freedom
Aside from well-known attribute amplifiers, gain-bandwidth independence, amplifiers also tend offer:
Gain
Faster Slew Rates Lower Distortion Feedback Restrictions
With these common attributes known, design engineer still ask: "Why?" This article will examine basics amplifier comparison with amplifier. following aspects each topology will examined:
Figure Basic Inverting Non-inverting Gain Topologies Hold True Amplifiers These transfer functions assume ideal conditions. Under ideal conditions, open loop gain A(s) amplifier open loop transimpedance gain Z(s) amplifier infinite. Therefore, ideal transfer function, non-inverting topology, generated follows:
Closed loop characteristics Open loop characteristics Input stage differences advantages
Once these aspects examined, will become apparent amplifiers have better specifications amplifiers have higher bandwidths same power better linear phase performance over wider bands. Finally, internal look amplifier will explain distortion slew rate enhanced topology. Closed Loop Characteristics basic amplifier design schematics their equations hold true both amplifier topologies. Figure shows basic circuit topologies transfer functions inverting non-inverting gain configurations. These hold true both amplifiers. point remember that value feedback resistor limited amplifiers. amplifier data sheet will provide recommended value.
VinG output equal input multiplied gain,
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Open Loop Characteristics fundamental differences between amplifiers begin show when comparing their open-loop characteristics. Figure illustrates open-loop characteristics amplifier.
A(s) A(s)
actual open loop gain large rolls rate octave, through most frequency range. frequency increases, value A(s) decreases. When A(s) overall gain circuit will half value. This commonly referred -3dB bandwidth amplifier. rate which bandwidth decreases proportional 1/G. most frequency range, product gain bandwidth becomes constant. This referred gainbandwidth product (GBP). prevents amplifiers from obtaining high gain high bandwidths simultaneously. This illustrated Figure
Gain
Figure Open-loop Characteristics ideal open-loop terminal characteristics are:
Infinite non-inverting inverting input
impedances Zero output impedance output voltage source that controlled potential difference between input terminals amplifier, also called error voltage V2). output equal this error voltage multiplied open loop gain, A(s). Once loop closed, feedback will attempt drive error voltage zero, hence term voltage feedback. Gain Bandwidth Product Refer non-inverting gain topology Figure Remember that open loop gain non-ideal amplifier finite. Reevaluating, non-ideal transfer function amplifier becomes: (Vin
Gain
Gain Bandwidth Bandwidth
Figure Open-loop Gain A(s) Illustration Amplifiers Open-Loop Characteristics Figure illustrates open-loop characteristics amplifier.
Vnon-inv
Z(s)
Z(s) Ierror
Vinv
Ierror
Figure Open-loop Characteristics There unity-gain buffer between inputs amplifier. Ideally, this buffer infinite input impedance zero output impedance. Therefore, ideal open-loop terminal characteristics are:
long A>>G then denominator becomes amplifier behaves ideal case.
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Infinite non-inverting input impedance Zero inverting input impedance Zero output impedance
output voltage source controlled error current, Ierror, inverting input. Once loop closed, feedback will attempt drive error current zero, hence term current feedback.
Gain Bandwidth Independence amplifiers known their gain-bandwidth independence. reason this attribute explained calculating transfer function non-ideal amplifier. evaluation transfer function non-inverting configuration shown. transfer function inverting configuration also illustrates gain-bandwidth independence. Ierror
input offset voltage (Vio) Matched input bias currents (Ib) High power supply rejection ratio (PSRR) Good common mode rejection ratio (CMRR)
close look input stages both topologies will explain amplifiers tend have better specifications.
Z(s) Ierror Z(s) Z(s) Z(s) Z(s) Z(s)
Vnon-inv
Vinv
Itail
Figure Typical Input Stage structure input stage reason better specifications. input stage often simple differential pair, identical bipolar transistors same bias current voltage. This configuration often called balanced circuit because symmetry between inputs. Because this symmetry, there will input offset voltage unless devices match. inputs bases transistors. Although absolute base currents, input bias currents, vary considerably process variation temperature, again unless devices identical, input bias currents will match. When either supply voltage common mode input voltage altered, change collector emitter voltages matched both input transistors. Changes devices bias point could effect offset, again balanced topology, bias currents match offset voltage little effected. result this good CMRR PSRR. Internal Look Topology input stage amplifier will also describe inherent traits amplifier:
transfer function looks very similar transfer function. long Z>>Rf then amplifier behaves ideal case. Once Z(s) drops where equals then gain lowered value. This differs from case where gain determined both amplifiers, gain increased lowering rather than increasing then bandwidth independent gain. This expression explains importance amplifiers. amplifier data sheets provide recommended values various gain settings. excessively large small will compromise stability. Within reason, feedback resistor used adjust frequency response. rule thumb, value recommended doubled, then bandwidth will half. Internal Look Topology observing open-loop characteristics both amplifier topologies, differences begin become apparent. However, closer look input stages will shed more insight into battle, VFB. typical amplifier input stage shown Figure common fact that amplifiers tend have better specifications than amplifiers. Most amplifiers have:
Nonzero Unmatched
input stage typical amplifier illustrated Figure voltage buffer. offset voltage zero, transistors would have match PNP. Since these devices
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constructed differently, there reason they would inherently match. Bias currents amplifiers also fundamentally mismatched. non-inverting bias current difference between base currents where inverting bias current depends errors produced next stage.
Current Mirror
Vnon-inv Ierr
Vinv
Vnon-inv Ierr
Vinv
Current Mirror
Figure Basic Topology Slew Rate Slew rate performance also enhanced topology. Refer typical topology Figure slew rate determined rate which second transistors charge compensation capacitors, current that sourced these transistors dynamic. limited fixed value often case topologies. With step input overload condition, current flowing through transistors increased overdriven condition quickly removed. first order, there slew rate limit this architecture. Some amplifiers have input structures similar amplifiers order take advantage higher slew rate possibilities. combination higher bandwidths slew rate allows devices have respectable distortion performance while doing lower power. basic current feedback amplifier fundamental slew-rate limit. Limits only come about parasitic transistor capacitances many strides have been made reduce even their effects. availability high-speed operational amplifiers both topologies allows design engineers select best amplifier his/her needs. amplifier compliments application that requires high slew rates, distortion, ability gain bandwidth independently. While amplifier compliments application where offset voltage noise specifications required.
Figure Typical Amplifier Input Stage Advantages Topology hidden advantage current feedback amplifiers that they usually require fewer internal gain stages than their voltage feedback counterparts. Often current feedback amplifier consists merely input buffer, gain stage output buffer. Having fewer stages means less delay through open-loop circuit. This translates into higher bandwidths same power. basic topology Figure single-stage amplifier. only high impedance node circuit input output buffer. amplifiers usually require more stages sufficient loop-gain. These additional stages delay yield lower stable bandwidths. Distortion distortion amplifier impacted openloop distortion amplifier overall speed closed-loop circuit. amount open-loop distortion contributed amplifier small basic symmetry topology. Figure illustrates typical topology. every transistor, there complimentary transistor. Speed other main contributor distortion. many gain configurations, amplifier greater bandwidth than counterpart. given signal frequency, faster part greater loop-gain therefore lower distortion.
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Current Voltage Feedback Amplifiers Debbie Brandenburg Janua

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