SL521 Log Amplifier

150MHz WIDEBAND LOG AMPLIFIER

SL521C Wideband Amplifier

FEATURES

  • Well-defined Gain
  • 4 dB Noise Figure
  • High I/P Impedance
  • Low O/P Impedance
  • 165 MHz Bandwidth
  • On-Chip Supply Decoupling
  • Low External Component Count
  • Logarithmic IF Strips with Gains up to 108dB and
    Linearity better than 1dB

DESCRIPTION

The SL521A, B and C are bipolar monolithic integrated circuit wideband amplifiers, intended primarily for use in successive detection logarithmic IF strips, operating at centre frequencies between 10MHz and 100MHz. The devices provide amplification, limiting and rectification, are suitable for direct coupling and incorporate supply line decoupling. The mid-band voltage gain of the SL521 is typically 12dB (4 times).
The device is also available as the 5962-90792 which has guaranteed operation over the full Military Temperature Range and is screened to MIL-STD-883 Class B.

APPLICATIONS
Logarithmic IF strips with gains up to 108 dB and linearity better than 1 dB.

OPERATING NOTES

The amplifiers are intended for use directly coupled.
The seventh stage in an untuned cascade will be giving virtually full output on noise.
Noise may be reduced by inserting a single tuned circuit the chain. As there is a significant mismatch between stages, simple shunt or series circuit cannot be used. The choice network is also controlled by the need to avoid distorting the logarithmic law; the network must give unity voltage transfer. The value of C must be chosen so that at resonance its admittance equals the total loss conductance across the tuned circuit Resistor R1 may be introduced to improve the symmetry of filter response, providing other values are adjusted for unity gain at resonance.
A simple capacitor may not be suitable for decoupling the output line if many stages and fast rise times are required. Alternative arrangements may be derived, based on the parasitic parameters given. Values of positive supply line decoupling capacitor required for untuned cascades are given below. Smaller values can be used in high frequency tuned cascades.
Number of stages:          6        5        4      3
Minimum capacitance:  30nF  10nF  3nF  1nF
The amplifiers have been provided with two earth leads avoid the introduction of common ground lead inductance between input and output circuits. The equipment designer should take care to avoid the subsequent introduction of such inductance.
The 500pF supply decoupling capacitor has a resistance of, typically, 100. It is a junction type having a low breakdown voltage and consequently the positive supply current will increase rapidly if the supply voltage exceeds 7.5V