Low Phase Noise Design: Isolation Amplifiers

155 Pages·2014·14.75 MB·English

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Low Phase Noise Design: Isolation Amplifiers (Bruce Griffiths)

Using wideband opamps as RF isolation amplifiers
When using an opamp as a low noise isolation amplifier, minimise the dc and low frequency noise gain of the opamp as
this minimises the flicker phase modulation of the amplifier output. Power supply noise also modulates the opamp
internal junction capacitances, etc., so a low noise power supply is essential for low flicker phase noise operation. Since
the Johnson noise of feedback and input resistors in the circuit adds to the phase noise floor, keeping the vale of these
resistors low minimises their contribution to the phase noise floor. Where cermet resistors (e.g. pots) are used then it is
essential to minmise the dc current flowing in these components otherwise they generate considerable excess noise at
low frequencies. Plastic, thick film and carbon resistors also generate excess noise when dc current flows in them, for
lowest noise metal film resistors should be used. Opamps should be selected for low noise, particularly in the flicker
region as well as adequate power bandwidth. In general current feedback opamps are unsuitable (at least for low gain
stages) as the noise voltage developed by by the inverting input noise current flowing in the feedback resistor can be
fairly large, particularly at low frequencies.

Dual supply opamp RF isolation amplifiers

Opamp unity gain RF isolation amplifier
The circuit below illustrates a dual supply RF amplifier using a fixed gain opamp . The input transformer steps up the
signal by 6dB to minimise the RF gain required. A noninverting amplifier is used as it has the lowest equivalent input
noise for low RF gains. C102 ensures that the opamp dc gain and low frequency gain is low minimising the low frequency
noise voltages across internal junctions and hence the opamp self noise flicker phase modulation. The feedback network
and input protection resistors values are low to keep their contribution to the noise floor well below that of the opamp.
C102 also ensures that the dc current flowing in the feedback resistor network is very low.
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Opamp variable gain RF isolation buffer
The circuit below illustrates a dual supply variable gain RF amplifier using an opamp. The input transformer steps up the
signal by 6dB to minimise the RF gain required. A noninverting amplifier is used as it has the lowest equivalent noise for
low RF gains. C102 ensures that the opamp dc gain and low frequency gain is low minimising the low frequency noise
voltages across internal junctions and hence the opamp self noise flicker phase modulation. The feedback network and
input protection resistors values are low to keep their contribution to the noise floor well below that of the opamp.
C102 also ensures that the dc current flowing in the feedback resistor network is very low so that a cermet pot can be
used in the feedback network without adding significant excess noise.
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Single supply opamp RF isolation amplifiers

Opamp fixed gain RF isolation amplifier
The circuit below illustrates a single supply fixed gain RF amplifier using an opamp. The input transformer steps up the
signal by 6dB to minimise the RF gain required. A noninverting amplifier is used as it has the lowest equivalent noise for
low RF gains. C104 ensures that the opamp dc gain and low frequency gain is low minimising the low frequency noise
voltages across internal junctions and hence the opamp self noise flicker phase modulation. The feedback network and
input protection resistors values are low to keep their contribution to the noise floor well below that of the opamp.
C104 also ensures that the dc current flowing in the feedback resistor network is very low. R101, R102, E101, C102 are
used to generate a low noise voltage divider to bias the transformer secondary and opamp noninverting input at 1/2 the
supply voltage.
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Opamp variable gain RF isolation amplifier
The circuit below illustrates a single supply variable gain RF amplifier using an opamp. The input transformer steps up
the signal by 6dB to minimise the RF gain required. A noninverting amplifier is used as it has the lowest equivalent noise
for low RF gains. C104 ensures that the opamp dc gain and low frequency gain is low minimising the low frequency noise
voltages across internal junctions and hence the opamp self noise flicker phase modulation. The feedback network and
input protection resistors values are low to keep their contribution to the noise floor well below that of the opamp.
C104 also ensures that the dc current flowing in the feedback resistor network is very low so that a cermet pot can be
used in the feedback network without adding significant excess noise. R101, R102, E101, C102 are used to generate a
low noise voltage divider to bias the transformer secondary and opamp noninverting input at 1/2 the supply voltage.
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Opamp unity gain RF isolation buffer
The circuit below illustrates a single supply unity gain RF amplifier using an opamp. The input transformer steps up the
signal by 6dB to minimise the RF gain required. A noninverting amplifier is used as it has the lowest equivalent noise for
low RF gains. The opamp noise gain is unity minimising the low frequency noise voltages across internal junctions and
hence the opamp self noise flicker phase modulation. The feedback network and input protection resistors values are
low to keep their contribution to the noise floor well below that of the opamp. R101, R102, E101, C102 are used to
generate a low noise voltage divider to bias the transformer secondary and opamp noninverting input at 1/2 the supply
voltage.

Discrete component RF isolation amplifiers
The lowest phase noise RF isolation amplifiers use discrete components rather than opamps. These RF isolation
amplifiers can have a phase noise floor 10dB or more below that achievable using opamps. However it can be difficult to
achieve low distortion unless power hungry differential amplifiers are used. Distortion can occur due to variation of
current gain with current, modulation of the output capacitance of a stage by large signals. If the signal is converted to a
current using a common base stage then by cascoding the input stage twice with additional common base stages the
reverse isolation can be very high and the distortion of the emitter current of the output stage can be -60dBc or less.
Unless the output stage collector load is kept low the output signal distortion may be as high as -40dBc unless a
differential common base output stage is used.

Low input impedance low input distortion, high output distortion, direct coupled common base cascade isolation
amplifier
In the circuit below Q101 reduces input distortion significantly by correcting for the nonlinearity of the emitter
impedance of Q102. However the high impedance load at Q104 collector increases the output distortion. The noise floor
can be typically reduced by 2dB to 3dB if Q102, Q103, Q104 are replaced by appropriately biased darlington pairs.
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Low input impedance, low input distortion, low noise direct coupled common base cascade isolation amplifier
In the circuit below Q101 reduces input distortion significantly by correcting for the nonlinearity of the emitter
impedance of Q102. However the high impedance load at Q104 collector increases the output distortion. The
transformer feedback from the collector of Q102 is quieter than the equivalent resistive series at the emitter. The noise
floor can be typically reduced by 2dB to 3dB if Q102, Q103, Q104 are replaced by appropriately biased darlington pairs.
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Low input impedance Low distortion common base cascade isolation amplifier with a low noise high isolation transformer feedback low distortion input
stage
In the circuit below Q101 reduces distortion significantly by correcting for the nonlinearity of the emitter impedance of Q102. The low imedance at the collector
of Q104 ensures that the output distortion is relatively low. Transformer feedback from the collector of Q102 is quieter than the equivalent series resistive
feedback at the emitter. The noise floor can be typically reduced by 2dB to 3dB if Q102, Q103, Q104 are replaced by appropriately biased darlington pairs.
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REFERENCES
1) A New 5 and 10MHz High isolation Distribution Amplifier
2) Origin of 1/f PM and AM Noise in Bipolar Junction Transistor Amplifiers
3) Guidelines for designing BJT Amplifiers with Low 1/f PM and AM Noise
4) A Low Noise 100MHz Distribution Amplifier for Precision Metrology
5) PM Noise Generated by Noisy Components
6) A Practical Guide to Isolation Amplifier Selection
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Development of a low noise high reverse isolation low distortion RF Amplifier

From: http://www.ko4bb.com/~bruce/CE TransformerFeedback BufferAmplifier.html

CE transformer feedback buffer amplifier

In this example the dc collector current of the RF transistor is stabilised by the pnp transistor which senses the voltage
drop across R and compares it with the ~1V reference formed by the difference between the LED forward voltage drop
and the Vbe of pnp transistor. The RF gain is largely determined by the transformer output to emitter turns ratio
(voltage gain ~ n/2) whilst the resistor in series with the emitter is selected to set the output impedance. The amplifier
works best when the gain is small (<10dB or so). For isolation amplifier applications one can cascade several such low
gain amplifiers to achieve the required reverse isolation as long as sufficient care is taken to minimise coupling via the
power supply. With a suitable RF transistor output levels of +27dBm or more can be achieved if the dc collector current
is high enough. Since some of the output current us supplied by the emitter circuit and the feedback is effective in
reducing distortion the dc collector current can actually be a little smaller than the peak RF load current without
significantly increasing the distortion.