REV. C

AD846

–9–

Figure 39. AD846 Three-Terminal Model

Figure 40. Op Amp Three-Terminal Model

A more detailed examination of the closed-loop transfer func-

tion of the AD846 results in the following equation:

Closed-Loop Gain G(s) =

−R

F

1

+ C

COMP

+ 1+



















s













Compare this to the equation for a conventional op amp:

Closed-Loop Gain G(s) =

−R

F

1

+

1

+









s









In the case of the voltage amplifier, the closed-loop bandwidth

closed-loop gain. However, for the transimpedance amplifier,

by the internal compensation capacitance of 7 pF and the value

ventional operational amplifier in which the input stage transcon-

ductance is equal to the inverting terminal input impedance of

Ω).

A simple equation can, therefore, be used to determine the band-

width of an amplifier employing the AD846 in the inverting

configuration.

3 dB Bandwidth

=

23

R

()

where: The 3 dB bandwidth is in MHz

G is the closed-loop inverting gain of the AD846

Ω.

10 k

1 k

Ω the bandwidth should be estimated from Figure 41.

Figure 41 illustrates the closed-loop voltage gain vs. frequency

of the AD846 for various values of feedback resistor. For com-

parison purposes, the characteristic of a conventional amplifier

having an 80 MHz unity gain bandwidth is also shown.

Figure 41. Closed-Loop Voltage Gain vs. Bandwidth for

of –10), the closed-loop bandwidth is approximately 28 MHz. It

normal practice for stabilizing conventional op amps, will cause

this amplifier to become unstable because the closed-loop band-

width will increase beyond the stable operating frequency.

A similar approach can be taken to calculate the noise perfor-

mance of the amplifier. A simplified noise model is shown in

Figure 42.

The equivalent mean-square output noise voltage spectral den-

sity will equal:









2

+ 4 kT R

F

+1







