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DETAILED DESCRIPTION

The GB4550(A) is intended for video applications requiring

coarse DC restoration coupled with flat frequency response.

As shown in Figure 1, the signal path features a wide band

operational amplifier designed to be unity gain stable. While

this amplifer is not intended to drive 75

Ω transmission lines,

it is ideal for applications where high capactive loads, up to

several hundred picofarads, must be driven, such as input

buffering and DC restoration of video signals.

Optimal frequency response for the GB4550(A) occurs with

load capacitances in the range of 80 pF to 100 pF as shown in

Figure 4. For smaller loads, an external capacitor can be

added to extend the bandwidth and improve the flatness of the

device response.

The clamping function is achieved through the use of a simple

comparator. The inverting input of the comparator is connected

to the GB4550(A) output, while the non-inverting input is

connected to the clamp voltage reference. For output signal

voltages more positive than the clamp reference the comparator

output is essentially open-circuit, while signal voltages more

negative than the clamp reference result in the charging of C

X

.

The action of the comparator is to provide a positive current

which is fed back to the op-amp non-inverting input under

conditions where the op-amp output is more negative than the

clamp reference voltage. This negative feedback raises the

DC level of the input signal to the point where all signal

fluctuations occur at voltages above the clamp reference

level. This is the desired clamp action.

The input to the op-amp must be AC coupled using an

appropriate size of capacitor, which then acts as a DC "reservoir"

for the corrective level shift.

Under equilibrium conditions the average current supplied by

the comparator output is just sufficient to balance the current

discharging the input capacitor. This discharge current is

simply the input bias current of the op-amp, typically less than

20

µA . However, an external resistor can be added to increase

the pull down current. Under dynamic conditions, where the

system is adjusting for a change in the signal level, the

charging current may be in the milliamp range. Because the

corrective current is small under equilibrium conditions, the

error voltage at the comparator input is small also, so clamping

accuracy to within

±7 mV is achievable.

The clamp circuit makes use of a "peak hold" capacitor, C

X

, at

the output of the comparator . This gives rise to a more

constant voltage at the comparator output which is translated

to a more constant corrective current by an internal 100 k

Ω

resistor connected between the comparator output and the

signal input.

To avoid excessive phase shift and consequent instability of

the clamp feedback loop, the peak hold capacitor needs to be

considerably smaller (e.g. 1000 times) than the input coupling

capacitor. If a faster clamp is desirable (e.g. for 60 Hz hum

elimination) the peak hold capacitor can be removed and a

smaller input coupling capacitor employed. In this application

some distortion of the signal "tip" is unavoidable.

+IN

10pF

COMP

OUT

30k

100k

7mA

Nominal

+

-

-IN

30k

(+2 V OUTPUT CLAMP)

+

-

(NC on

GB4550A)

All resistors in ohms, all capacitors in microfarads unless otherwise stated.

Fig. 1 Simplfied Circuit Diagram