AD846 datasheet(9/12 Pages) AD

9 / 12 page

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

+ C

COMP

+ 1+









RIN











s













Compare this to the equation for a conventional op amp:

Closed-Loop Gain G(s) =

−R

CCOMP

















where: CCOMP is the internal compensation capacitor of the am-

plifier; gM is the input stage transconductance of the amplifier.

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

decreases directly with increasing values of (1 + RF/RS), the

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

the situation is different. At low gains, where (1 + RF/RS) RIN is

small compared to RF, the closed-loop bandwidth is controlled

by the internal compensation capacitance of 7 pF and the value

of RF, and not by the closed-loop gain. At higher gains, where (1

+ RF/RS) RIN is much larger than RF, the behavior is that of a con-

ventional operational amplifier in which the input stage transcon-

ductance is equal to the inverting terminal input impedance of

the transimpedance amplifier (RIN = 50

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

F + 0.05 1 + G

()

where: The 3 dB bandwidth is in MHz

G is the closed-loop inverting gain of the AD846

RF is the feedback resistance in k

Ω.

NOTE: This equation applies only for values of RF between

10 k

Ω and 100 kΩ, and for RLOAD greater than 500 Ω. For RF =

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

Various Values of RF

For the case where RF = 1 k

Ω and RS = 100 Ω (closed-loop gain

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

should also be noted that the use of a capacitor to shunt RF, a

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:

VON

2 = R

F I NN

()2 + 1+ RF









[VN

2 + R

P INP

()2 +4 kT R

P ]

+ 4 kT R









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