The use of low-cost buffer-amplifier ICs can provide the output
isolation needed to improve the frequency stability and noise
performance of a VCO. Isolation between two circuit stages is
often achieved by introducing a buffer amplifier between them.
This isolation minimizes adverse effects in a wide range of
applications, and especially for those using RF signals. Poor
output isolation can produce various mechanisms that disturb
oscillator performance, so a buffer stage is often included at
oscillator outputs. In discussing these uses for buffer
amplifiers, this article focuses on a voltage-controlled
oscillator (VCO) whose active device is an NPN bipolar
transistor. The resulting applications generally operate with
other active devices as well. Before discussing the addition of
a buffer-amplifier stage, consider some typical VCO
characteristics and how isolation problems can degrade
performance. A common problem, for example, is the frequency
change exhibited by a VCO in response to varying load conditions