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EL4093C

300 MHz DC restored video amplifier

Elantec -> Intersil 

EL4093C

300 MHz DC-Restored Video Amplifier

Applications Information Contd

Autozero Range

The autozero range is defined as the difference

between the input DC level and the reference

voltage to restore to The size of this range is a

function of the gain setting resistors used and the

S H output current swing For a gain of a2 the

optimum feedback resistor is 750X and the avail-

able S H output current is g5 5 mA minimum

To determine the autozero range for this case we

refer to Figure 3 below

Figure 3

4093 – 39

Suppose that the input DC level is aVDC and

that the reference voltage is 0V We know that in

feedback the following two conditions will exist

on the CFA first its output will be equal to 0V

(due to autozero) and second its VINb voltage

is equal to the VINa voltage (i e VINb e

aVDC) So we have a potential difference of

aVDC across both RF and RG resulting in a cur-

rent IRF e IRG e VDC 750X that must flow

into each of them This current IAZ e (IRF a

IRG) must come from the S H output Since the

maximum that IAZ can be is 5 5 mA we can solve

for VDC using the following

 J IAZ e g5 5 mA e 2

VDC

750X

and see that VDC e g2V This range can easily

accommodate most video signals

As another example consider the case where we

are restoring to a reference voltage of a0 75V

Using the same reasoning as above a current IRF

e (VDC b 0 75V) RF must flow through RF

and a current IRG e VDC RG must go into RG

Again our boundary condition is that IRF a IRG

s g5 5 mA and we can solve for the allowable

VDC values using the following

g5 5 mA e VDC b 0 75V a VDC

750X

750X

Hence VDC must be between a2 4V to b1 7V

This example illustrates that when the reference

changes the autozero range also changes In gen-

eral the user should determine the autozero

range for his her application and ensure that the

input signal is within this range during the au-

tozero period

Autozero Loop Bandwidth

The gain-bandwidth product (GBWP) of the au-

tozero loop is determined by the size of the hold

capacitor the value of RF and the transconduct-

ances (gm’s) of the S H amplifier To begin the

S H amplifier is modeled as in Figure 4 below

First the input stage transconductance is repre-

sented by gm1 with the compensation capacitor

given by CHOLD This stage’s GBWP is thus

gm1 (2q  CHOLD) e 1 (2q  (350X)(2 2 nF))

e 207 kHz Next since the S H has a current

output its output stage can be modeled as a

transconductance gm2 in this case having a val-

ue of 1 (500X) The current from gm2 then flows

through the I to V converter made up of the CFA

and RF to produce a voltage gain Thus the

GBWP of the overall loop is given by

gm1

GBWP e 2q  CHOLD (gm2  RF)

11

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