# Transistor radio amplifier noise in the 70s

• Ephant
Ephant
In the 1970s, did transistor radios have RF receive sensitivities down to the 1 microvolt level? If not, what is the highest microvolt it could resolve?

But how could it do that? Even modern amplifiers have much noises. For example, at 100kHz, and a very quiet circuit with total noise of 5nV/Sqrt (Hz) that we couldn't even create now. The noise is 5nV/Sqrt (Hz) x Sqrt (100kHz BW) = 5nV x 316.23 = 1581 nV or 1.58 uV (microvolt). It could drown the 1uV signal. But in the 1970s, how can transistor radio have RF receive sensitivities down to the 1 microvolt level? (what is the highest then if it is not 1uV? )

Transistor radios for AM have a bandwidth of 10 kHz.
√10k = 100.
100 * 5 nV = 500 nV = 0.5 uV.

Baluncore said:
Transistor radios for AM have a bandwidth of 10 kHz.
√10k = 100.
100 * 5 nV = 500 nV = 0.5 uV.

This formula is to compute for noise. So a radio set with total noise of 5nV/Sqrt (Hz) with 10kHz bandwidth has noise rms of 0.5uV. In the 1970s. Were there radio with total noise this low? Or better yet. In the 1970s. What is the lowest noise amplifier available? Were there 1nV/Sqrt (Hz) amplifer similar to modern ones or were noises in the 1970s in the 50nVSqrt (Hz)? I want to see list of noises of amplifiers from the 1960s up to the 2020s out of curiosity to know how much they can resist white noise and manufacturing techniques to lower them.

Somewhere in my workshop I've got these:
HP465A, circa 1965, with 25uV over 1MHz BW (RTI), for white noise, that's equivalent to 25nV/√Hz.
HP461A, circa 1963, with 40uV over 150MHz BW (RTI), equivalent to 3.3nV/√Hz.

These aren't heroic circuits, just good lab preamps. Granted, they aren't radios, but...

Even today, if you want really low noise, you might put a big discrete JFET in front of your low noise op-amp, like they did back in the day. This is a case where (junction) size matters.

berkeman
DaveE said:
Somewhere in my workshop I've got these:
HP465A, circa 1965, with 25uV over 1MHz BW (RTI), for white noise, that's equivalent to 25nV/√Hz.
HP461A, circa 1963, with 40uV over 150MHz BW (RTI), equivalent to 3.3nV/√Hz.

These aren't heroic circuits, just good lab preamps. Granted, they aren't radios, but...

Even today, if you want really low noise, you might put a big discrete JFET in front of your low noise op-amp, like they did back in the day. This is a case where (junction) size matters.

Let's take the example of the JFET OPA2132P which is used as input stage for the main amp AMP01. The OPA2132P (with 8nV/Sqrt (Hz) noise is a JFET input amplifier and as such has very high impedance, very low bias current and very low current noise. This allows it to have consistent performance even with a very high source impedance. Hence it is used as input stage for the AMP01 which has more bias current and more current noise. In the 1960s. Do their input stage and main amps have similar noises as it is now? So the noises are the same compared to the 1960s and only the chips get smaller?

Btw.. for FM radio, what is the smallest microvolt the receiver needs to be able to resolve it?

Ephant said:
for FM radio, what is the smallest microvolt the receiver needs to be able to resolve it?
Umm.. What? Are you trolling us?
Wouldn't that depend on how close you are to the transmitter, how powerful the transmitter is, etc. ?
There's lots of stuff on the web about low noise analog design, maybe you should study some of that stuff.
I'm not really motivated to search for and read data sheets for you. I think I'm done here.

Ephant said:
0.5uV. In the 1970s. Were there radio with total noise this low? Or better yet.
The sensitivity of a domestic radio receiver is not really critical when natural atmospheric noise, QRN, and other man-made interference, QRM, dominate the broadcast band.

For Radio Astronomy, cooling amplifiers to near absolute zero reduced the noise.
In the 1970s, FET amplifiers became available, with lower noise than BJTs.

Before that, parametric amplifiers were used for RA.
https://en.wikipedia.org/wiki/Parametric_oscillator#Parametric_amplifiers

So it means if your radio receiver only has 1mV sensitivity, then you can only receive signal with 1mV and if your radio receiver has 1uV sensitivity, then you can receive very weak signal? Is that it? I didn't know that. I thought all radio either has fixed 1uV or 1mV signal for example like direct input signal.

Ephant said:
I thought all radio either has fixed 1uV or 1mV signal
Logic should tell you that the received signal level would depend on many factors - distance from transmitter would be a major one.

You seem to be in a bit of an information vacuum so you'd benefit from few visits to an amateur radio website where everything relevant is discussed at many different levels. Just use your searching skills; the journey can be as valuable as actually arriving.

Bystander
sophiecentaur said:
Logic should tell you that the received signal level would depend on many factors - distance from transmitter would be a major one.

You seem to be in a bit of an information vacuum so you'd benefit from few visits to an amateur radio website where everything relevant is discussed at many different levels. Just use your searching skills; the journey can be as valuable as actually arriving.

Yes I got it. The electrons going up and down the antenna is what creates the uV or mV signal at the ampliifer. I was thinking of a pure wave detector and my experience with circuit are always signal generators.

Anyway. For vacuum tubes. What is the lowest nV/Sqrt (Hz) noise in the amplifier. And also transistor amplifiers. Can someone give a list of white noises of amplifiers down the decades and timeline of the improvements and technology?

Ephant said:
Can someone give a list of white noises of amplifiers down the decades and timeline of the improvements and technology?
Sounds to me like an excellent piece of potential personal research for the OP. Without some basic knowledge of the subject, all a list would provide would be a list of numbers.

nsaspook
sophiecentaur said:
Sounds to me like an excellent piece of potential personal research for the OP. Without some basic knowledge of the subject, all a list would provide would be a list of numbers.

I built audio amplifier kits.. I just don't build any radio as radio is not my interest. Someone just told me that radio equiptments in the 1970s can be sensitive down to 1 uV and wrote this thread to ask if true. I'm interested in amplifiers overall from the 1950s up to now.

Ephant said:
Yes I got it. The electrons going up and down the antenna is what creates the uV or mV signal at the ampliifer. I was thinking of a pure wave detector and my experience with circuit are always signal generators.

Anyway. For vacuum tubes. What is the lowest nV/Sqrt (Hz) noise in the amplifier. And also transistor amplifiers. Can someone give a list of white noises of amplifiers down the decades and timeline of the improvements and technology?
One of the receivers I used and repaired was the R-1051. A odd-ball solid-state receiver with two tubes in the RF end for signal overload immunity (transmitters were usually close to the receiver sites/antennas) and EMP protection (the tube would flash-over but would be expected to recover and still work). The RF noise and sensitivity stats are not stellar but we used them for world-wide HF communications links on ships and shore radio stations because man-made and atmospheric noise was a much larger factor than internal noise.

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Ephant said:
Someone just told me that radio equiptments in the 1970s can be sensitive down to 1 uV
You need to be careful with this sort of 'evidence'. An adequate service level for sound VHF FM would be around 5mV into 50Ω (see what the first level of a Google search gives you) Stereo reception requires higher signals than mono. The problem with fm receotion is that there is a threshold effect, below which the signal just dies. Compare this with AM which keeps going as the signal level reduces and your brain has to drag information out a a hissy signal on a comms channel.

You'd need to poke around in google an find some good data to fit what you actually want to know.

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