Figure 319 No gain compensation in an amplifier in Software

Creator GS1 - 12 in Software Figure 319 No gain compensation in an amplifier

Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website

cies, and with a high enough feedback path somewhere on the board, oscillation can become a problem if the layout is poor In addition, the higher the gain of an amplifier stage, the more likely oscillations will begin to break out; 25 dB of gain is considered the maximum for stability from a single stage Neutralization (degenerative feedback), as mentioned above, is sometimes used to stabilize a potentially unstable amplifier Nonetheless, the amplifier neutralization procedure will be successful only if the positive feedback path that created instability and oscillations is internal to the transistor, and not if poor layout and/or lack of input/output shielding creates the return path Neutralization is also problematic with wideband transistor amplifiers because of the variations in input and output capacitance of a bipolar transistor with changes in frequency and bias currents, as well as the neutralization retuning requirements for transistors in different production lots Another viable technique for creating a stable amplifier is to simply reduce the gain of the stage This works because an amplifier, as mentioned above, must reach the Barkhausen criterion to oscillate (just as an oscillator must) This means that reducing the feedback and/or the gain will stabilize an amplifier Unfortunately, reducing stage gain appreciably is often an unacceptable solution, both from an economic and an efficiency standpoint

Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website

gain and stability, we may not want to be as formal with our calculations as we have been up to now, because of time constraints Ballpark figures will sometimes suffice There is a much faster method to obtain gain and stability figures, called S-parameter scalar approximations, that can be utilized for amplifiers to obtain approximate design values For the following formulas, only the magnitudes of the S parameters are employed, and not the phase angles 1 Gtu (transducer unilateral gain) is the dB measurement of an amplifier s power gain into an unmatched 50-ohm load a worst-case gain value and can be roughly calculated by: Gtu 10 log10 |S21|2

2 Mismatch losses ( p) at the transistor s input or output, in dB, are calculated by: pIN pOUT 10 log10 (1 10 log10 (1 S112) S222)

Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website

4 Since designing for MAG is never recommended because an amplifier can be unstable at this high gain value, we would like to be able to compute the MSG, or the maximum stable gain: MSG 10 log10 (|S21| |S12|)

Thus, if the MAG is smaller than the MSG, then the amplifier will be unconditionally stable, unless poor circuit layout produces an external feedback path Scalar approximation is a more rapid technique than the methods presented in the prior pages, since we are employing only the magnitude of the S parameters, and not their phase angle As an example, we are given a transistor with the following S parameters: S11 S22 S12 S21 0195 1676 0508 32 0139 612 25 624 0195, S22 0508, S21 25, S12 0139, and

which demonstrates that about 146 dB will be gained by proper impedance matching MAG 796 dB 146 dB 942 dB

(1063 dB was calculated for this same transistor with the full MAG method described earlier) MSG 10 log10 (|25| |0139|) 1255 dB