KH563 Datasheet, PDF(8/13 Page) Cadeka Microcircuits LLC. – Wideband, Low Distortion Driver Amplifier

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KH563 Datasheet, PDF (8/13 Pages) Cadeka Microcircuits LLC. – Wideband, Low Distortion Driver Amplifier

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DATA SHEET

KH563

recognize that [taking Vi positive]

Vo = Vâ + Gierr Rf

solving for Vâ from two directions

Vâ = Vi â ierr Ri = (G + 1) ierr Rg

solving for ierr from this

ierr

(G + 1) Rg

+ Ri

then

Vâ

Vi Ri

(G + 1) Rg

+ Ri

and, substituting for Vâand ierr in the original Vo expression

= Vi

ï£®

ï£¯1+

ï£°ï£¯

GRf â Ri

(G + 1) Rg +

ï£¹

ï£º

ï£»ï£º

pulling an Rf out of the fraction

â¡

= 1+ Rf

ï£®

ï£¯

G â Ri

ï£¯

ï£°

ï£¹

ï£º

ï£»

note that Av

ï£«ï£ï£¬

G+

1ï£¶ï£¸ï£·

Ri = 0

Figure 4: Voltage Gain Derivation

Note again that if Ri = 0 this expression would simplify

considerably. Also, if G were very large the voltage gain

expression would reduce to the familiar non-inverting op

amp gain equation. These two performance equations,

shown below, provide a means to derive the design equa-

tions for Rf and Rg given a desired no load gain and out-

put impedance.

Performance Equations

Design Equations

ï£«

ï£ï£¬1+

G + 1+ Ri

ï£¶

ï£¸ï£·

= 1+ Rf

ï£®

ï£¯

G â Ri

ï£¯

ï£°ï£¯

ï£¹

ï£º

ï£»ï£º

Rf = (G + 1) Ro â Av Ri

Rf â Ro

Av â1

Equivalent Model

Given that the physical feedback and gain setting

resistors have been determined in accordance with the

design equations shown above, an equivalent model may

be created for the gain to the load where the

amplifier block is taken as a standard op amp. Figure 5

shows this analysis model and the resulting gain

equation to the load.

Classical

op-amp

Rf - Ro

ï£«

ï£ï£¬1+

Rf â Ro

ï£¶

ï£¸ï£·

RL + Ro

KH560 Fig 5

substituting in for Rf and Rg with their design

equation yields

RL + Ro

(gain to load)

Figure 5: Equivalent Model

This model is used to generate the DC error and noise

performance equations. As with any equivalent model,

the primary intent is to match the external terminal

characteristics recognizing that the model distorts the

internal currents and voltages. In this case, the model

would incorrectly predict the output pin voltage swing for

a given swing at the load. But it does provide a simplified

means of getting to the external terminal characteristics.

External Compensation Capacitor (Cx)

As shown in the test circuit of Figure 1, the KH563 requires

an external compensation capacitor from the output to

pin 19. The recommended values described here assume

that a maximally flat frequency response into a matched

load is desired. The required Cx varies widely with

the desired value of output impedance and to a lesser

degree on the desired gain. Note from Figure 2, the

simplified internal schematic, that the actual total

compensation (Ct) is the series combination of Cx and

the internal 10pF from pin 19 to the compensation nodes.

The total compensation (Ct) is developed in two steps as

shown below.

300

ï£«

ï£¬1â

ï£

2.0ï£¶

ï£·

ï£¸

pF intermediate equation

(0.02)

pF total compensation

REV. 1A January 2008

▷