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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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5th Sep 2010, 9:57 am | #1 |
Nonode
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
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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? |
5th Sep 2010, 10:32 am | #2 |
Guest
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Re: Transistor Choice in radio project
Google them...
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5th Sep 2010, 11:39 am | #3 |
Moderator
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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. |
5th Sep 2010, 12:22 pm | #4 |
Octode
Join Date: Jul 2008
Location: Gloucestershire, UK.
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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 Last edited by Mr Moose; 5th Sep 2010 at 12:32 pm. |
5th Sep 2010, 10:54 pm | #5 |
Nonode
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
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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. |
6th Sep 2010, 12:05 am | #6 |
Retired Dormant Member
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Location: Stoke-on-Trent, Staffordshire, UK.
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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. Last edited by f.cooper; 6th Sep 2010 at 12:21 am. |
6th Sep 2010, 10:10 am | #7 |
Nonode
Join Date: Jul 2005
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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. |
6th Sep 2010, 10:21 am | #8 |
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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. |
6th Sep 2010, 10:47 am | #9 |
Retired Dormant Member
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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. |
6th Sep 2010, 12:27 pm | #10 |
Dekatron
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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 |
6th Sep 2010, 12:58 pm | #11 | |
Dekatron
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Re: Transistor Choice in radio project
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. 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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6th Sep 2010, 1:08 pm | #12 |
Retired Dormant Member
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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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6th Sep 2010, 1:28 pm | #13 | ||
Dekatron
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Re: Transistor Choice in radio project
Quote:
Obviously we've no idea what the OP is doing... Quote:
Mark |
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6th Sep 2010, 1:40 pm | #14 |
Dekatron
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Location: Evesham, Worcestershire, UK.
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Re: Transistor Choice in radio project
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6th Sep 2010, 2:00 pm | #15 |
Retired Dormant Member
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Re: Transistor Choice in radio project
If ft is 300kHz then at 10kHz hfe can be no greater than 30.
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6th Sep 2010, 2:22 pm | #16 |
Dekatron
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
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Re: Transistor Choice in radio project
Yup, which implies an hie @10KHz of ~900 ohms at 1mA collector current. Good job it's an AM set
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