THS4601_14 Datasheet, PDF (12/32 Pages) Texas Instruments – WIDEBAND, FET-INPUT OPERATIONAL AMPLIFIER

THS4601
SLOS388B − OCTOBER 2001 − REVISED JUNE 2002
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
by the input bias currents, I_IB, of the amplifier.
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
feedback resistor, R_F. The transfer function from input to output is V_OUT = I_INR_F.
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
RF
_
λ
+
RL
−VBias
Figure 26. Wideband Photodiode Transimpedance Amplifier
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