Page 152 - Complete Wireless Design

Amplifier Design

Amplifier Design 151

safety from oscillation in high-gain amplifiers from VHF upward will normally
demand some small value of resistive collector loading. The higher the value
of the resistor, the more stability, but the less amplifier gain. Values of
between 5 to 20 ohms should suffice.
As an example of LNA design, the ideal bias point for the lowest NF for a
certain low-noise transistor was found to be V 10 V and I 6 mA on the
CE C
device’s data sheet. The optimal for low-noise is and equals 0.65 138°
S S(opt)
for this transistor, also as printed on the data sheet, for the frequency and bias
of interest ( may likewise be referred to as and ). Along with ,
S(opt) 0 opt S(opt)
we will also find the G expected of the transistor at our operating frequency;
a
this is the associated gain at the minimum noise figure, in dB. The S parame-
ters of the transistor at the above bias conditions are S 0.35 160°; S
11 22
0.37 36°; S 0.05 61°; S 3.4 62°. Design an LNA that has a Z of
12 21 IN
50 ohms and a Z of 50 ohms at 500 MHz. Calculate impedance matching
OUT
networks and gain.
1. Is the transistor unconditionally stable (K 1)?
2. The for optimal NF, as stated on the data sheet, is 0.65 138°, which
S(opt)
equals 0.48 j0.43.
3. Find the input matching network’s optimum NF matching from source to
the transistor’s input using in step 2; make the transistor’s input
S(opt)
think that it is seeing as its source.
S(opt)
4. Find
S S
21
12
S(opt)
(S )
L 22 1 S
11 S(opt)
where load reflection coefficient
L
value as shown in step 2 above
S(opt)

complex conjugate of the result
Use vector quantities for S.
5. Match the transistor’s output to the next stage.

3.2 Large-Signal Amplifiers
3.2.1 Introduction
Linear Class A power and small-signal amplifiers can be designed by S param-
eters. Nonlinear, Class B and C power amplifiers cannot reliably exploit these
parameters, but instead must depend mainly on large-signal input/output
parameter design. These values can be found in the power transistor’s data
sheet in rectangular notation (such as 1.1 j3.2) for its series input and out-
put impedances at a number of frequencies, and at a specific V and P . The
CC OUT
series input and output impedances can be made available as a Smith chart

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