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Buffer Amplifiers
CE transformer feedback buffer amplifier
When one is using an OCXO like an HP10544A which is has a minimum specified load impedance of 1K and wishes to drive a 50 ohm load with a +13dBm signal then a low noise buffer amplifier with a 50 ohm output impedance and a high input impedance combined with a reverse isolation of at 30dB or more is usually required. The Buffer amplifier depicted below uses transformer(1:4:2 turns ratio) feedback from the collector to the emitter to achieve this. It has a nominal voltage gain from input to output of 0dB (set by the transformer collector winding to emitter winding turns ratio) with an output impedance of 50 ohms set by R8 and a reverse isolation of more than 40dB at 10MHz. The amplifier is also suitable for boosting the output of a 10811A to +13dBm. Q2, R4 and the RED LED D1 set the dc collector current of Q1. The phase noise floor of this amplifier with a 10MHz +13dBm output should be around -178dBc/Hz at offsets of 1kHz or more.
If one has an OCXO (e.g. E1938A) or GPSDO (e.g. Z3815) with an output of around +4dBm the amplifier shown below can be used to boost the output level to +13dBm. The reverse isolation is > 40dB and the phase noise floor with a 10MHz +13dBm output is below -170dBc/Hz for offsets of 1kHz or more.
The above circuits are intended for operating frequencies of 20MHz and below, although they should work at 100MHz with reduced output and higher phase noise. Operation at frequencies up to several hundred MHz is possible if suitable transistors are substituted for the 2N3904's and the supply voltages are lowered to accommodate the lower voltage ratings of transistors with fts of several GHz.
Design Details
For more detail on the design and development of common emitter transformer feedback amplifiers see: Common Emitter Transformer feedback amplifier design
REFERENCES
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