THS4601

SLOS388B – OCTOBER 2001 – REVISED JUNE 2002

12

www.ti.com

APPLICATION INFORMATION

introduction

The THS4601 is a high-speed, FET-input operational amplifier. The combination of its high frequency

capabilities and its DC precision make it a design option for a wide variety of applications, including test and

measurement, optical monitoring, transimpedance gain circuits, and high-impedance buffers. The applications

section of the data sheet discusses these particular applications in addition to general information about the

device and its features.

transimpedance fundamentals

FET-input amplifiers are often used in transimpedance applications because of their extremely high input

impedance. A transimpedance block accepts a current as an input and converts this current to a voltage at the

output. The high-input impedance associated with FET-input amplifiers minimizes errors in this process caused

designing the transimpedance circuit

Typically, design of a transimpedance circuit is driven by the characteristics of the current source that provides

the input to the gain block. A photodiode is the most common example of a capacitive current source that would

interface with a transimpedance gain block. Continuing with the photodiode example, the system designer

traditionally chooses a photodiode based on two opposing criteria: speed and sensitivity. Faster photodiodes

cause a need for faster gain stages, and more sensitive photodiodes require higher gains in order to develop

appreciable signal levels at the output of the gain stage.

These parameters affect the design of the transimpedance circuit in a few ways. First, the speed of the

photodiode signal determines the required bandwidth of the gain circuit. However, the required gain, based on

the sensitivity of the photodiode, limits the bandwidth of the circuit. Additionally, the larger capacitance

associated with a more sensitive signal source also detracts from the achievable speed of the gain block. The

dynamic range of the input signal also places requirements on the amplifier’s dynamic range. Knowledge of the

source’s output current levels, coupled with a desired voltage swing on the output, dictates the value of the

The large gain-bandwidth product of the THS4601 provides the capability for achieving both high trans-

impedance gain and wide bandwidth simultaneously. In addition, the high power supply rails provide the

potential for a very wide dynamic range at the output, allowing for the use of input sources which possess wide

dynamic range. The combination of these characteristics makes the THS4601 a design option for systems that

require transimpedance amplification of wideband, low-level input signals. A standard transimpedance circuit

is shown in Figure 26.

_

+

CF

λ

RL

RF

–VBias

Figure 26. Wideband Photodiode Transimpedance Amplifier