Preamp with only one transistor or with opamp IC?The formula for spectral density of noise voltage generated by a resistance is u°=rt(4kTR) or expressed as square of voltage u°^2=4kTR.
For room temperature T=300K the formula becomes u°^2=1.6E-20*R.
Since resistance of RX coil generates noise, we can make a table like this:
Coil resistance - Spectral density of noise voltage
10 ohm - 0.405 nV/rtHz
20 ohm - 0.566 nV/rtHz
50 ohm - 0.905 nV/rtHz
100 ohm - 1.28 nV/rtHz
200 ohm - 1.79 nV/rtHz
500 ohm - 2.83 nV/rtHz
1000 ohm - 4.05 nV/rtHz
2000 ohm - 5.66 nV/rtHz
Note that when we increase coil resistance twice, the noise not increases twice, but only sqrt2=1.41 times. The notation u° means RMS value. When we calculate peak-to-peak density of noise voltage u p-p, we should multiply the RMS value by 6 to 8 as was shown visual in other postings.
If we compare with above table the noise density of NE5534 given in data sheet u°=4 nV/rtHz, we will see that this IC adds to input so much noise as a 1000 ohm coil or resistor. However our coil has much lower resistance, for example 50 ohm. Remember that a low noise transistor generates noise with its rbb'<50 ohm.
When we use IC operational amplifier, we should connect to its inverting input a noise generating resistor noted as R1 in the attached circuit diagram. How to calculate resistance of R1?
In ideal case, if the IC is noiseless, we simply can choose R1<rs and problem is solved easy. However with an IC which generates noise as a 1000 ohm resistor, we can use R1 almost 500 ohm and despite R1>rs, this will not increase significant input noise.
If we build differential amplifier with PNP low noise transistors, this will increase current drain and noise in compariso with only one transistor
CONCLUSION: If we can compensate AIR and GND signals in input of RX, it is preferable to add a low noise preamp builded with only one PNP transistor.
The next question is:
How to design the resistance of RX coil for a low noise front end?