Current feedback op-amps

References: National Semiconductor application notes: OA15, AN-597Texas Instruments: OpAmps for Everyone, Chapter 8: CFOAIntersil Elantec EL-5166 datasheet

What is a CFOA?

A CCII- , a voltage follower, and a node impedance

How did the idea come about?

Supply mirroring: CC Supply mirroring + Buffer

Current conveyor Current feedback op-amp

Loop gain calculation

Basic usage: Non inverting amplifier

( )F GOUTNINV

IN G F B G B F G

Z Z ZdVGdV ZZ Z Z Z Z Z Z

+= =

+ + +

Note that we have not introduced explicit frequency dependence!

lim 1 FNINVZ

G

ZGZ→∞

= +

Not common usage: Inverting

GINV −ZZFZZGZBZFZBZGZFZG

lim FINVZ

G

ZGZ→∞

= −

Gain calculation – frequency response

GNINV ZZFZZGZZGZBZFZBZGZFZG

ZT Z0

1s1 1s2

ZB hib RB01

1s0T1sT /11/0

Transimpedance ZT has two poles: due to the impedance of the high Z nodeand also due to the current mirror delay ZB is the output impedance of the input buffer, small but not necessarily much smaller than ZG

Gain calculation – frequency response (2)

GNINV ZZFZZGZZGZBZFZBZGZFZG ZT

Z01s1 1s2

Neglect ZB since it is much smaller than anything else. Then,

( ) 111 /

F G FNINV

G F G G F

BandwidthGain

Z Z Z ZZZZ Z Z Z Z Z

+ ⎛ ⎞= +⎜ ⎟+ +⎝ ⎠

Observe that the gain and bandwidth have been decoupled. The bandwidth depends only on ZF (and ZB, really), while the gain depends on both ZF and ZG

There is no magic involved. The CFOA is a dominant pole transimpedance amplifier, while the voltage and current buffers have poles at much higher frequencies.

Gain vs. frequency with RF (EL5166)

Gain vs frequency with RG (EL5166)

Notice that despite what our approximations say the BW still depends on gain!

Gain bandwidth product – example

Frequency response of transimpedance

The transimpedance has a dominant pole at 100kHz

Slew rate: VFOA vs CFOA

• VFOA:

• CFOA:

0

0.5

1

1.5

2

2.5

3

3.5

4

0 2 4 6 8 10

In the limit of large input voltages,in the VFOA:

While in the CFOA we get:

The slew rate in the CFOA is power limited

0out

in

dIdV

→

outout

in

dI IdV

→∝

( )tanhoutVFOAI V V+ −∝ −

( )expoutCFOA yI V∝

Re-visit the non inverting amplifier

The CFOA is a transimpedance amplifier, i.e. equivalent to the block diagramThe response is, in terms of the transimpedance Z is the usual FB expression:

VIN

YG+YF Z

YF

VOUT

( ) lim 1 11

G F G FNINV NINVZ

F F G

Z Y Y Y ZG GZY Y Z→∞

+= ⇒ = + = +

+Notice that the loop gain is ZZF and that Z is typically second order beyond the first pole, so that, for a parallel RC in the feedback path we get a second order underdamped system.

Avoid capacitances!

Effect of Feedback capacitance

INSTABILITY unless we introduce extra pole!

Input capacitance

Brute force compensation

Low pass the non-inverting input to compensate for stray capacitance,i.e. brute force compensation

Effect of input (stray) capacitance

Output (load) capacitance

Applications I

Inverting receiver Non-inverting receiver

Applications II

Differential amplifier Differential line driver

Antenna circuits

Coax cable driver Distribution amplifier

CFOA can be combined with other stages…

With emitter follower

With op-amps

Bandwidth and stability

Adjust BW by increasing inverting input impedance!

Integrator

Integrating capacitance NOT in feedback path. Capacitance must be lossy enough!

Implementation note: Supply bypassing is NOT optional!