THS4601

SLOS388B – OCTOBER 2001 – REVISED JUNE 2002

16

www.ti.com

APPLICATION INFORMATION

measuring transimpedance bandwidth (continued)

VS

50

Ω

50

Ω

RS

C1

C2

Network Analyzer

IO

I

O

V

S

+

1

2R

S

1

)

C

1

C

2

(above the pole frequency)

NOTE: This interface network creates a capacitive, constant current source from a network analyzer and properly terminates the network analyzer

at high frequencies.

Figure 32. Emulating a Capacitive Current Source With a Network Analyzer

The transconductance transfer function of the interface circuit is

I

O

Vs

(s)

+

s

2R

S

1

)

C

1

C

2

s

)

1

2R

S

C

1

2

.

This transfer function contains a zero at DC and a pole at s

+

1

2R

S

C

1 )

C

2

. The transconductance is

constant at

1

2R

S

1

)

C

1

C

2

, above the pole frequency, providing a controllable AC current source. This circuit

also properly terminates the network analyzer with 50

Ω at high frequencies. The second requirement for this

current source is to provide the desired output impedance, emulating the output impedance of a photodiode or

other current source. The output impedance of this circuit is given by

Z

O

(s)

+

C

C

s

)

1

s s

)

1

.

1

sC

2

,

giving the appearance of a capacitive source at higher

frequency.

transconductance network is much less than the transimpedance bandwidth of the circuit. Choosing this corner

frequency properly leads to more accurate measurements of the transimpedance bandwidth. If the interface

circuit’s corner frequency is too close to the bandwidth of the circuit, determining the power level in the flatband

is difficult. A decade or more of flat bandwidth provides a good basis for determining the proper transimpedance

bandwidth.