Designers can build a 15-dB compressor with a miniature lamp and a current-feedback amplifier. The circuit possesses extremely low distortion at frequencies above the lamp's thermal time constant. This means that distortion is negligible from audio frequencies to beyond 10 MHz. There's also relatively little change in phase versus gain compared to other automatic gain-control circuits. Lastly, the circuit has many instrumentation, audio, and high-frequency applications as a result of its low distortion and small phase change.
The AD844 op amp is a perfect fit for this application because it's a current-feedback amplifier. Each stage of the circuit, U2, lamp, and feedback resistor compresses an ac signal by over 15 dB (see the fig-ure). Cascading a number of stages delivers higher compression ranges.
Op amp U1 operates as a unity-gain buffer to drive the input to the compressor. However, U1 is optional if a low-impedance signal source is used. The lamp's resistance will increase with temperature, which reduces the ratio of resistor R3 to the resistance of the lamp. This ratio reduces the gain of U2. The lamp's cold resistance should be greater than the input resistance of U2 (more than 50Ω) for proper oper-ation. The lamp's resistance will change slightly for low input levels. Therefore, the ratio of R3 to the resistance of the lamp and the gain of U2 stays high.
The AD844 is a high speed monolithic operational amplifier fabricated using Analog Devices'junction isolated complementary bipolar (CB) process. It combines high bandwidth and very fast large signal response with excellent dc performance. Although optimized for use in current to voltage applications and as an inverting mode amplifier, it is also suitable for use in many noninverting applications.
The AD844 can be used in place of traditional op amps, but its current feedback architecture results in much better ac performance, high linearity and an exceptionally clean pulse response.
This type of op amp provides a closed-loop bandwidth which is determined primarily by the feedback resistor and is almost independent of the closed-loop gain. The AD844 is free from the slew rate limitations inherent in traditional op amps and other current-feedback op amps. Peak output rate of change can be over 2000 V/ms for a full 20 V output step. Settling time is typically 100 ns to 0.1%, and essentially independent of gain. The AD844 can drive 50 W loads to ±2.5 V with low distortion and is short circuit protected to 80 mA.
The AD844 is available in four performance grades and three package options. In the 16-pin SOIC (R) package, the AD844J is specified for the commercial temperature range of 0 to +70. The AD844A and AD844B are specified for the industrial temperature range of 40 to +85 and are available in the cerdip (Q) package. The AD844A is also available in an 8-pin plastic mini-DIP (N). The AD844S is specified over the military temperature range of 55 to +125. It is available in the 8-pin cerdip (Q) package. "A" and "S" grade chips and devices processed to MIL-STD-883B, REV. C are also available.
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Wide Bandwidth: 60 MHz at Gain of 1 33 MHz at Gain of 10Very High Output Slew Rate: Up to 2000 V/s20 MHz Full Power Bandwidth, 20 V pk-pk, RL = 500 Fast Settling: 100 ns to 0.1% (10 V Step)Differential Gain Error: 0.03% at 4.4 MHzDifferential Phase Error: 0.158 at 4.4 MHzHigh Output Drive: 650 mA into 50 W LoadLow Offset Voltage: 150 mV max (B Grade)Low Quiescent Current: 6.5 mAAvailable in Tape and Reel in Accordance with EIA-481A Standard
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Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .±18 V
Power Dissipation2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 W
Output Short Circuit Duration . . . . . . . . . . . . . . . . . Indefinite
Common-Mode Input Voltage . . . . . . . . . . . . . . . . . . . . . . ±VS
Differential Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . 6 V
Inverting Input Current
Continuous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 mA
Transient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 mA
Storage Temperature Range (Q) . . . . . . . . . . 65 to +150
(N, R) . . . . . . . . 65 to +125
Lead Temperature Range (Soldering 60 sec) . . . . . . . . +300
ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1000 V
Post a Buying Lead
Required Field 
