EL4093C 查看數據表（PDF） - Elantec -> Intersil

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EL4093C

300 MHz DC restored video amplifier

Elantec -> Intersil 

EL4093C

300 MHz DC-Restored Video Amplifier

Applications Information Contd

Figure 4

4093 – 40

With RF e 750X a GBWP of 310 kHz is ob-

tained Note however that this is the small signal

GBWP As mentioned earlier the sample and

hold has special boost circuits built in which pro-

vides g8 5 mA of charge current during full slew

These boost circuits turn on when the S H input

differential voltage exceeds g50 mV When the

boosters are turned on gm1 greatly increases and

the circuit becomes nonlinear Thus some stabili-

ty issues are associated with the boosters and

they will be addressed in a later section

Charge Injection and Hold Step

Charge injection refers to the charge transferred

to the hold capacitor when switching to the

HOLD mode The charge should ideally be 0 but

due to stray capacitive coupling and other effects

is typically 0 1 pC in the EL4093 This charge

changes the hold capacitor voltage by DV e DQ

CHOLD and this DV is multiplied by the output

stage transconductance (gm2) to produce a

change in S H output current This last quantity

is listed as the spec DISTEP and is calculated us-

ing the following

 J DQ

DISTEP e CHOLD  gm2

For CHOLD e 2 2 nF and gm2 e 1 (500X)

DISTEP has a typical value of 100 nA This

change in S H output current flows through RF

shifting the CFA output voltage However as we

shall soon see this shift is negligible Assuming

RF e 750X DISTEP is impressed across RF to

give (750X)(100 nA) e 0 08 mV of change at the

CFA output

Droop Rate

When the S H amplifier is in HOLD mode there

is a small current that leaks from the switch into

the hold capacitor This quantity is termed the

droop current and is typically 10 nA in the

EL4093 This droop current produces a ramp in

the hold capacitor voltage which in turn produc-

es a similar effect at the CFA output The Droop

Rate at the CFA output can be found using the

equation below

Droop

e

IDROOP

CHOLD

(gm2



RF)

Assuming RF e 750X and CHOLD e 2 2 nF the

drift in the CFA output due to droop current is

about 7 mV ms Recall that in NTSC applica-

tions there is about 60 ms between autozero peri-

ods Thus there is 7 mV ms)(60 ms) e 0 4 mV or

less than 0 1 IRE of drift over each NTSC scan

line This drift is negligible in most applications

12

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