  We examine a silicon-germanium heterojunction bipolar transistor (HBT) for
cryogenic pre-amplification of a single electron transistor (SET). The SET
current modulates the base current of the HBT directly. The HBT-SET circuit is
immersed in liquid helium, and its frequency response from low frequency to
several MHz is measured. The current gain and the noise spectrum with the HBT
result in a signal-to-noise-ratio (SNR) that is a factor of 10-100 larger than
without the HBT at lower frequencies. The transition frequency defined by SNR =
1 has been extended by as much as a factor of 10 compared to without the HBT
amplification. The power dissipated by the HBT cryogenic pre-amplifier is
approximately 5 nW to 5 {\mu}W for the investigated range of operation. The
circuit is also operated in a single electron charge read-out configuration in
the time-domain as a proof-of-principle demonstration of the amplification
approach for single spin read-out.
