Input impedance into amplifier - mismatch

[[email protected]]

Apologies for what may seem to be a very obvious question, but I've
been struggling to understand exactly what is going on.

I have an output from a photodiode, picking up a beat signal between
two lasers of the order of 200Mhz. This signal is to be fed into a
chain of RF electronics, all Minicircuits components at 50ohm
impedance. Before this, I need to amplify the relatively weak signal
from the photodiode(~-34dBm) to around -5dBm, and to this end I have
purchased a Minicircuits ZFL series amplifier which should fit my gain
needs.

Question is, assuming the load of the amplifier is 50 ohms (the other
RF electronics)..how much effect does the input impedance have on the
eventual signal?

Impedances up to 5k from the photodiode circuit have been used
(improves sensitivity to certain frequencies), but I suspect that its a
pipedream to imagine this working with the amplifier.

Any help greatly appreciated!

Sam

 

[Winfield Hill]

[email protected] wrote...

I have an output from a photodiode, picking up a beat signal between
two lasers of the order of 200Mhz. This signal is to be fed into a
chain of RF electronics, all Minicircuits components at 50ohm
impedance. Before this, I need to amplify the relatively weak signal
from the photodiode(~-34dBm) to around -5dBm, and to this end I have
purchased a Minicircuits ZFL series amplifier which should fit my gain
needs.

Question is, assuming the load of the amplifier is 50 ohms (the other
RF electronics)..how much effect does the input impedance have on the
eventual signal?

Impedances up to 5k from the photodiode circuit have been used
(improves sensitivity to certain frequencies), but I suspect that its
a pipedream to imagine this working with the amplifier.

You'll pay a considerable Johnson-noise penalty if you stick with
low 50-ohm current-to-voltage signal impedances. Your 200MHz laser
application may be a good fit for Phil Hobbs' common-base amplifier
approach. Explore his web pages, http://users.bestweb.net/~hobbs/

Properly implemented, a common-base amplifier allows you to present
a low or 50-ohm impedance to high-frequency PD signals, while still
developing the signal output voltage across higher resistance values,
reducing the effect of Johnson noise, which goes as sqrt(4kT/R).

 

[tlbs]

I worked on a very similar project 2 years ago. We had an array of
heterodyne detectors measuring the beat frequency of a LASER and it's
time-delayed light. We were sampling the heterodyne signal at 2 GHz.

Each of the 16 detectors had their own individual preamplifier, the
inputs of which were matched to the detectors (high impedance). The
outputs of the preamplifiers were 50 Ohms, so then the rest of the
system was 50 Ohms. The gain of the preamplifiers was very high (on
the order of 1000) and the noise figure was impressive for room-temp
operation.

Unfortunately I don't recall the manufacturer of the preamps, and I no
longer have access to that info.

 

[dalits]

Apologies for what may seem to be a very obvious question, but I've
been struggling to understand exactly what is going on.

Try Transimpedance amplifiers.
That is what is used to match photo diodes to first amps, current to voltage
conversion mostly.

 

[Phil Hobbs]

Apologies for what may seem to be a very obvious question, but I've
been struggling to understand exactly what is going on.

I have an output from a photodiode, picking up a beat signal between
two lasers of the order of 200Mhz. This signal is to be fed into a
chain of RF electronics, all Minicircuits components at 50ohm
impedance. Before this, I need to amplify the relatively weak signal
from the photodiode(~-34dBm) to around -5dBm, and to this end I have
purchased a Minicircuits ZFL series amplifier which should fit my gain
needs.

Question is, assuming the load of the amplifier is 50 ohms (the other
RF electronics)..how much effect does the input impedance have on the
eventual signal?

Impedances up to 5k from the photodiode circuit have been used
(improves sensitivity to certain frequencies), but I suspect that its a
pipedream to imagine this working with the amplifier.

Any help greatly appreciated!

Sam

It all depends on the photocurrent and the noise figure of the amplifiers,
but generally speaking, you're right that 50-ohm amps are a poor choice.

What is the optical power level at the photodiodes?

Cheers,

Phil Hobbs

 

[[email protected]]

I have 3mW optical power at the minute, leading to photocurrents of
around 1-2mA.

I have researched into transimpedance amplifiers for the photodiode,
but the problem is finding op-amps which are relatively cheap,
non-surface mount (I would prefer to avoid the cost/inconvinience of
producing PCBs) and of sufficient bandwidth.

At the minute, careful photodiode selection has meant noise is fairly
minimal (I'm quite surprised for what is essentially a simple resistor,
current/voltage conversion) and should not infringe too much on my
intended application.

Cheers

Sam

 

[KoKlust]

At the minute, careful photodiode selection has meant noise is fairly

minimal (I'm quite surprised for what is essentially a simple resistor,
current/voltage conversion) and should not infringe too much on my
intended application.

Can you tell us what is actually the problem with your current solution?

If you want the best signal to noise ratio, you will need to use a
transimpedance amp since you can design its feedback network to match
optimally to the photodiode impedance.

Beware that if you want to do anything serious at 200MHz frequencies, you
MUST design a pcb to keep the parasitic capacitances and inductances low.

By the way you are lucky to receive so much light.

 

[Phil Hobbs]

I have 3mW optical power at the minute, leading to photocurrents of
around 1-2mA.

I have researched into transimpedance amplifiers for the photodiode,
but the problem is finding op-amps which are relatively cheap,
non-surface mount (I would prefer to avoid the cost/inconvinience of
producing PCBs) and of sufficient bandwidth.

At the minute, careful photodiode selection has meant noise is fairly
minimal (I'm quite surprised for what is essentially a simple resistor,
current/voltage conversion) and should not infringe too much on my
intended application.

A very useful rule is that shot noise dominates thermal noise when
2*e*I_dc > 4kT/R, i.e. when V_dc == I_dc*R > 2kT/e, 50 mV at room
temperature. Thus with a room-temperature resistive load, or in your
case an RF amp with a 3-dB noise figure, a photocurrent larger than 1 mA
will put you in the shot noise limit.

Noise temperature in RF amps is a very useful parameter when the source
is not impedance-matched, as photodiodes are not. Unlike the situation
in op amp TIAs, the noise temperature is not closely related to the
temperature of the resistor itself. You can get RF amps with noise
temperatures down to 35K at room temperature, or 10K at nitrogen
temperature (77K). Cooled varactor parametric amplifiers can go lower
still. This noise temperature is what you plug into the formula above,
so with a cooled amplifier, you could be in the shot noise limit with
only (2k/eR)*10K = 85 uA of photocurrent, even at 50 ohms.

Thus if your RF amps are ordinary common-or-garden ones with NF~3dB,
you're golden.

Cheers,

Phil Hobbs

 

[Winfield Hill]

Phil Hobbs wrote...

A very useful rule is that shot noise dominates thermal noise when
2*e*I_dc > 4kT/R, i.e. when V_dc == I_dc*R > 2kT/e, 50 mV at room
temperature. Thus with a room-temperature resistive load, or in your
case an RF amp with a 3-dB noise figure, a photocurrent larger than
1 mA will put you in the shot noise limit.

Noise temperature in RF amps is a very useful parameter when the source
is not impedance-matched, as photodiodes are not. Unlike the situation
in op amp TIAs, the noise temperature is not closely related to the
temperature of the resistor itself. You can get RF amps with noise
temperatures down to 35K at room temperature, or 10K at nitrogen
temperature (77K). Cooled varactor parametric amplifiers can go lower
still. This noise temperature is what you plug into the formula above,
so with a cooled amplifier, you could be in the shot noise limit with
only (2k/eR)*10K = 85 uA of photocurrent, even at 50 ohms.

Thus if your RF amps are ordinary common-or-garden ones with NF~3dB,
you're golden.

Unless his photodetector is unhappy with so much light, and his 1mA
exceeds the recommended maximum current-density for a spot of light.
Just something that should be checked at high light levels.