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Old 5th Sep 2010, 9:57 am   #1
Neil Purling
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Default Transistor Choice in radio project

A 3 transistor regenerative TRF: OC45 & 2x OC71.
A fellow constructor made the same circuit & introduced two changes: OA79 instead of OA91 diodes and also substituting OC81D for the pair of OC71's.
His effort sounded great & I wondered if this was due to the OC81D having greater gain than a OC71. Or maybe the diode swap had some effect?
Can anyone tell me what the gain of a OC71 and a OC81.OC81D should be from data sheets?
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Old 5th Sep 2010, 10:32 am   #2
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Default Re: Transistor Choice in radio project

Google them...
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Old 5th Sep 2010, 11:39 am   #3
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Default Re: Transistor Choice in radio project

The gain is a nominal figure and will vary quite a bit in different examples. An OC71 is a general purpose AF amp and an OC81D is specifically intended for use driving a pair of OC81s in a transistor radio, but they are really very similar transistors.

The audio stages in these homebrew designs will work with a very wide range of small signal Ge transistors.
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Old 5th Sep 2010, 12:22 pm   #4
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Default Re: Transistor Choice in radio project

Hello,
Towers gives the minimum Hfe (Common emitter current gain) for each type as follows:-

OC71 30 (minimum) @ 5mA bias

OC81 50 (minimum) @ 100mA bias

OC81D 20 (minimum) @ 5mA bias

I think there can be quite a variation in gain with different transistors of the same type.

Yours, Richard

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Old 5th Sep 2010, 10:54 pm   #5
Neil Purling
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Default Re: Transistor Choice in radio project

Having enjoyed most success with those glowing glass bottles, transistors are a bit of a mystery.
Would there be very wide variations in these old transistor types?
I just wonder how many examples of OC81 you'd have to go through to get particularly good examples.
I think the circuit before me may be a bit of a bodge. I would be quite happy to name it in a Private Message to comply with the Forum rules and gain useful advice.
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Old 6th Sep 2010, 12:05 am   #6
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Default Re: Transistor Choice in radio project

Hi,
Looking back through the Mullard data books (1965-1967) they kept moving the goal posts with regard to how hfe was expressed.

For the OC81 in 1966/67 it was hfe (Vce = -6v; Ic =1mA Typical 80

For the OC81 in 1965/66 it was hfe min ( Ic = 300mA ) 45

The OC81 were probably at least the third generation of low power audio O/P transistors.

The OC81D was specifically for use as a driver and consequently had lower current capability, lower hfe and also lower power.

The OC72s were amongst the first generation.

Later on the OC81s were phased out and replaced by the AC128 ( AC127 an NPN could be used to complement) , this enabled the driver and O/P transformers to be eliminated thus a cost saving.

As it has been pointed in another thread various audio packages ( LFK4 etc )were distributed where supposedly the output pair were matched.
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Old 6th Sep 2010, 10:10 am   #7
Neil Purling
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Default Re: Transistor Choice in radio project

f.cooper:
How do your Mullard reference books rate the hfe of the OC71 & OC75?

I bought a kit of bits. The kit is to build a radio from a circuit in a book.
The audio stage has the emitter of the first OC71 wired to the +ve rail, no emitter resistor.
I wondered why there wasn't something with its associated bypass capacitor.
The supplier of the kit has tweaked the circuit to add a 1K emitter resistor & 22uf bypass and employ OC81's in place of the two OC71 transistors.
I am trying a emitter resistor & bypass with the OC71's in case this makes an improvement without fitting OC81s.
If the audio stage is weak as originally planned with OC71's I will make note and move on based on what help I get here.
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Old 6th Sep 2010, 10:21 am   #8
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Default Re: Transistor Choice in radio project

There is nothing wrong with using OC71s in this position. They were intended for this sort of small signal AF amplification application.

You won't get a significant difference in performance by subbing any similar transistor from the Ge era. The variation in gain between transistors of the same type is much greater than the variation between the nominal hfe numbers of different types.

Differences in the performance of homebrew projects like this once built will be mostly caused by different construction techniques or even construction errors.
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Old 6th Sep 2010, 10:47 am   #9
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Default Re: Transistor Choice in radio project

Hi,
The OC71 had identical ratings in both the data books:-

hfe typ (Ic = 1mA, VCE - -2V ) 41

The OC71 was a first generation general purpose transistor for low power applications
( Pto max. 75mW ). It was generally OK for audio applications but no good for rf applications, OC45 (was used for IF frequencies ( 470 Kcs ) whereas the OC44 was used as a mixer ( frequency changer ). Most early transistors had limitations as to maximum useable frequency.

Figures quoted in most data books were generally the minimum or average values to expect.
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Old 6th Sep 2010, 12:27 pm   #10
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Default Re: Transistor Choice in radio project

Um, what has hfe got to do with the gain of the amplifier?

Transistors are transconductance amplifiers - voltage in, current out. This is proven by the Ebers-Moll model which describes the relationship of VBE to IC, and from which one can derive the simplified rules gain=gm.RL, where gm=35.Ic (or 40.Ic, depending on the operating temperature). You only need to consider hfe when establishing the biasing circuits.

Incidentally, AGC circuits work by changing the bias which changes collector current, which in turn changes gm.

Given how "shifty" hfe is (temperature, device sample, collector current, planetary alignment), it's a damn good job it's a pretty incidental parameter. When you do consider it, you always find the worst-case published value, then assume something well less again. Examples might be emitter-followers in audio output stages or pass transistors in power supplies.

In your case, if you get significantly different results from different transistors, assuming that the DC conditions remain identical (important!), then I would offer the following possible explanations:

1. Germanium transistors have quite high leakage, and this can be modelled by an internal resistance between collector-emitter. All transistors have this as a predictable part of their operation (it's called hoe), but the leakage appears in parallel with it. As these are in parallel with RL, the gain comes down... In extreme cases, the collector-emitter junction might be a few hundred ohms and there might be no transistor action at all - and depending on the surrounding circuit, the DC conditions might actually be OK (I last saw this on a Roberts R707 where TR4 (BC158) had failed. Actually, I've found the Lockfit transistors to be pretty unreliable for silicon...)

2. The frequency matters, and at audio, pretty much all transistors are the same, but at RF or even IF frequencies, you have to consider the inter-electrode capacitance. These vary between different transistor types, although are reasonably consistent between different samples of the same transistor...

Hope this helps,

Mark
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Old 6th Sep 2010, 12:58 pm   #11
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Default Re: Transistor Choice in radio project

Quote:
Originally Posted by mhennessy View Post
Um, what has hfe got to do with the gain of the amplifier?...You only need to consider hfe when establishing the biasing circuits.
Voltage gain is sensibly independent of hfe, agreed, but the input impedance of a common-emitter stage is highly dependent on hfe.

So, if you are using your transistor to amplify the output of a diode detector, having an output impedance of a few kilohms, you want an amplifier with a highish input impedance. Otherwise, it'll load the detector, the AF voltage will drop, the transistor will see a smaller input, and give a smaller output.
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Old 6th Sep 2010, 1:08 pm   #12
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Default Re: Transistor Choice in radio project

I think there may be a confusion between hFE(large signal) and hfe(small signal). hFE is relevant to setting the bias point. hfe will determine the input impedance for signals. hfe is strongly frequency dependent when the signals are a significant fraction of the cutoff frequency of the device. The hfe figure is quoted for low frequencies. At fT, hfe is unity by definition. For devices like the OC71 I think that hfe will be starting to fall noticeably in the audio band.
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Old 6th Sep 2010, 1:28 pm   #13
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Quote:
Originally Posted by kalee20 View Post
Voltage gain is sensibly independent of hfe, agreed, but the input impedance of a common-emitter stage is highly dependent on hfe.
Yes, hie is hfe/gm. For a typical silicon small-signal device at 1mA, you're talking about 5-8K. The bias network normally contributes as well. But of course, this is the case when there's no emitter resistor (normally a bad plan) or there's a decoupled emitter resistance (not great either - for audio at least - unless there's global NFB elsewhere); otherwise the input impedance is raised by the feedback factor. Of course, this is at LF where the "Millered" collector-base capacitance isn't bringing this down even further, and while I haven't done the sums, I would suspect that for a typical CE IF/RF amplifier, this might well dominate.

Obviously we've no idea what the OP is doing...



Quote:
So, if you are using your transistor to amplify the output of a diode detector, having an output impedance of a few kilohms, you want an amplifier with a highish input impedance.
Having recently studied lots of 1960s radios, I was surprised that the detectors where typically driving into ~5K - I certainly expected higher than that, and couldn't see any other reason for not using higher values. This leads me to conclude that other factors are at play, and I assume that the Hacker brothers et al knew far more than I ever will

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Old 6th Sep 2010, 1:40 pm   #14
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Quote:
Originally Posted by ppppenguin View Post
For devices like the OC71 I think that hfe will be starting to fall noticeably in the audio band.
Really

I must check Towers when I get home, but I was assuming an ft of a few hundred KHz. This implies a really high Cob...
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Old 6th Sep 2010, 2:00 pm   #15
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Default Re: Transistor Choice in radio project

If ft is 300kHz then at 10kHz hfe can be no greater than 30.
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Old 6th Sep 2010, 2:22 pm   #16
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Yup, which implies an hie @10KHz of ~900 ohms at 1mA collector current. Good job it's an AM set
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