THS4041, THS4042

165MHz CSTABLE HIGHSPEED AMPLIFIERS

SLOS237B− MAY 1999 − REVISED FEBRUARY 2000

13

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

theory of operation

The THS404x is a high-speed, operational amplifier configured in a voltage feedback architecture. It is built

using a 30-V, dielectrically isolated, complementary bipolar process with NPN and PNP transistors possessing

fTs of several GHz. This results in an exceptionally high performance amplifier that has a wide bandwidth, high

slew rate, fast settling time, and low distortion. A simplified schematic is shown in Figure 62.

IN − (2)

IN + (3)

NULL (1)

NULL (8)

(6) OUT

(4) VCC −

(7) VCC +

Figure 62. THS4041 Simplified Schematic

noise calculations and noise figure

Noise can cause errors on very small signals. This is especially true when amplifying small signals, where

signal-to-noise ration (SNR) is very important. The noise model for the THS404x is shown in Figure 63. This

model includes all of the noise sources as follows:

• e

n = Amplifier internal voltage noise (nV/√Hz)

• IN+ = Noninverting current noise (pA/√Hz)

• IN− = Inverting current noise (pA/√Hz)

• e

Rx = Thermal voltage noise associated with each resistor (eRx = 4 kTRx )