SL1521 Logarithmic Amplifier

Applications

• Radar IF Strips
• Wideband Amplification

The SL1521A and C are wideband amplifiers intended for use in successive detection logarithmic IF strips operating at centre frequencies of up to 200MHz. It is a plug-in replacement for the SL521 series of RF amplifiers. The mid-band voltage gain of the SL1521 is typically 12dB. The SL1521A and C differ mainly in the tolerance of voltage gain.

The amplifiers are intended for use directly coupled, as shown in Figure 8.
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 in the chain. As there is a significant mismatch between stages, a simple shunt or series circuit cannot be used. The choice of network is also controlled by the need to avoid distorting the logarithmic law; the network must give unity voltage transfer at resonance. A suitable network is shown in Figure 9. The value of C1 must be chosen so that at resonance its admittance equals the total loss conductance across the tuned circuit.
A simple capacitor may not be suitable tor decoupling the output line if many stages and fast rise times are required. Values of positive supply line decoupling capacitor required for untuned cascades are given below. Smaller values can be used in high frequency tuned cascades.
The amplifiers have been provided with two earth leads to avoid the introduction of common earth lead inductance between input and output circuits. The equipment designer should take care to avoid the subsequent introduction of such inductance.