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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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Old 16th May 2019, 3:25 pm   #1
martin.m
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Default Transistor gain

Is a transistor's hfe number a measurement of it's gain and if an I.F. amp transistor in an AM radio was replaced by a transistor with a much higher hfe number, should the radio's RF performance be improved? I checked the hfe of two transistors, an AF117 which gave a reading of 131 and a BC557 which measured 426. Trying them both in a Hacker AM only radio, there appeared to be no difference in performance. This is just an example, obviously the AF117 would be less reliable in the long term.
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Old 16th May 2019, 3:42 pm   #2
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Default Re: Transistor gain

The quoted hfe figure is under specific conditions and a simple comparison of completely different types isn't meaningful. Good circuit design also doesn't rely on specific hfe characteristics.
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Old 16th May 2019, 5:44 pm   #3
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Default Re: Transistor gain

The hfe figure is the current gain of a transistor, and as Paul says it's under specific conditions. When looking at hfe figures, pay close attention to the collector current and voltage it was specified under. It can change quite a lot, especially with collector current. Really good datasheets give graphs of hfe versus current.

hfe tells you nothing about the frequency capabilities. ft is the frequency at which the falling current gain with frequency is projected to hit unity.

hfe tells you nothing about the noise behaviour

hfe tells you nothing about optimum source and load impedances the device should be presented with for best gain and lowest noise (sadly usually different impedances!) These figures become rather important in RF circuits.

hfe itself varies a lot from device to device even of the same type number. Good designers make circuits which will tolerate this. Plenty of devices have open ended specs with no maximum quoted. So them both working similarly isn't a surprise, just an indication of good design.

Note that your little hfe meter measured the two transistors at different currents.

Too many people get too precious over the closeness of equivalents. Quite often you can get away with murder, but proving it from design calculations is a lot of work.

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Old 17th May 2019, 3:30 am   #4
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Default Re: Transistor gain

I still have a transistor tester I built from a blank sheet and a photo copy of a data sheet with those Ic and Hfe curves. It was in the 1970s.
I ended up with 5 fixed resistors for the collector and variable base supply with points to connect a 20K per volt meter.
It has a compariter based on a metal canned 741 driving a pair of early LEDs. This gives fixed Vce for all readings.
Each measurement had to be calculated to get Hfe.
Curve tracers were huge and expensive at the time.
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Old 17th May 2019, 4:05 pm   #5
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Default Re: Transistor gain

Keep an eye out for a Heathkit IT-1121 Semiconductor Curve Tracer.

It connects up to any Oscilloscope with an X-Y facility and is a very useful thing to have around, especially if you want to match pairs of transistors.

They turn up occasionally at radio rallies and sell for for 10 or so.
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Old 17th May 2019, 6:22 pm   #6
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Default Re: Transistor gain

One of the old hands at Plessey once remarked that some designs that worked fine with early production transistors were prone to oscillate with later batches of the same type that had higher gains/frequency responses. The usual cure was to slip a small ferrite bead over one of the transistor legs.
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Old 17th May 2019, 6:25 pm   #7
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Default Re: Transistor gain

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Originally Posted by emeritus View Post
One of the old hands at Plessey once remarked that some designs that worked fine with early production transistors were prone to oscillate with later batches of the same type that had higher gains/frequency responses. The usual cure was to slip a small ferrite bead over one of the transistor legs.
It's a known issue: power-supplies using the first 2N3055 series-pass transistors could sometimes turn into impressive medium-wave power-oscillators if you replaced failed series-pass transistors with later 2N3055s whose HF gain was higher as a result of improved manufacturing processes.
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Old 17th May 2019, 7:35 pm   #8
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Default Re: Transistor gain

22pF across collector-base junction does a good job of making a good transistor a rubbish one again. Adds miller capacitance.
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Old 17th May 2019, 8:25 pm   #9
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Default Re: Transistor gain

Thank you for your replies and comments. The subject is not as straightforward as I thought. The component tester was bought from Ebay for under 15. It's an improvement on checking transistor junctions with a multimeter and will also measure inductance, capacitance and resistance (up to about 150k).
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Old 18th May 2019, 1:49 am   #10
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Default Re: Transistor gain

These cheap component testers are very useful and I use a similar one myself, but they are often confused by Ge devices, I guess because of the inherent leakage.
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Old 18th May 2019, 2:13 am   #11
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Default Re: Transistor gain

Quote:
Originally Posted by martin.m View Post
Thank you for your replies and comments. The subject is not as straightforward as I thought. The component tester was bought from Ebay for under 15. It's an improvement on checking transistor junctions with a multimeter and will also measure inductance, capacitance and resistance (up to about 150k).
I have got a tester the same as yours.
Have you tried the remote control test function yet?
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Old 18th May 2019, 4:04 am   #12
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Default Re: Transistor gain

One thing it pays to remember about germanium transistors, take the AF127 for example, they have somewhat fragile junctions a max base current for example of only 1mA and a max collector current of 10mA. On the other hand for a BC557 and many modern silicon signal types a max base current peak value of 200mA and a peak collector current of 200mA and an average collector current of 100mA, so they are very hardy to any tester protocols and tests with meters. The designers of modern transistor/fet testers often have not laid eyes on a vintage germanium transistor.

So it is worth knowing if your tester will or will not stress a germanium signal transistor.

As noted the hfe is a wildly varying parameter and good circuit design means the circuit will work with a wide range of hfe values. I would avoid though replacing a transistor type with one that has a much higher fT. For example for an AF127, its 75MHz, but for a BC557 it is 200MHz. Sometimes its possible with this for a lower frequency radio stage to go into VHF oscillation, just due to the inductance of wires and tracks if a very lively high fT device is put in there.

In addition, although you will find you can drop a BC557 in, in place of an AF117 or AF127 and it "works" does not mean it is working as well as it could. Due to the higher base-emitter voltage of the silicon type, the base and collector currents for the stage you drop it into will be lower than for the germanium case, and ideally the bias resistor should be lowered for optimum performance of the silicon type in a germanium's physical circuit.
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Old 18th May 2019, 6:50 pm   #13
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Default Re: Transistor gain

Argus25 thanks for the advice. I wouldn't normally use a BC557 as an IF amp and prefer to replace germanium with the same in transistor radios. I think it may have been you that recommended using AF178s in a previous post and I now have a little stockpile of them. They work well in AM Hackers and don't look out of place.

Refugee, I didn't know the tester could check remote controls. It came with a little bag containing an LED and a capacitor. I have no idea what they are for as there were no instructions.
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Old 19th May 2019, 1:38 am   #14
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Default Re: Transistor gain

You just point the remote at it and push the test button without a component connected so that the remote can see the little IR logo and it shows the codes the remote sends to the telly.
You have to push a button on the remote before it reports that there is no component connected quite obviously.
The banggood website shows an internal view and even then I wondered why I could see a remote sensor on the board. I never scrolled down the description far enough before deciding it did enough for me to buy it.
I investigated later and found out it did remotes as well.
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Old Yesterday, 10:32 pm   #15
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Default Re: Transistor gain

Quote:
Originally Posted by G6Tanuki View Post
Quote:
Originally Posted by emeritus View Post
One of the old hands at Plessey once remarked that some designs that worked fine with early production transistors were prone to oscillate with later batches of the same type that had higher gains/frequency responses. The usual cure was to slip a small ferrite bead over one of the transistor legs.
It's a known issue: power-supplies using the first 2N3055 series-pass transistors could sometimes turn into impressive medium-wave power-oscillators if you replaced failed series-pass transistors with later 2N3055s whose HF gain was higher as a result of improved manufacturing processes.
We had similar at GEC. Due to the bean counters, we often found that one supply/MANUFACTURER of a component ( usually an IC) had dried up ,to be replaced with an other make, along with known problems.
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Old Today, 1:46 am   #16
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Default Re: Transistor gain

One classic was the Royer oscillator or DC DC converter typical of the early 1960's used to generate HT from 12V supplies in amateur radio gear and in CD ignitions being some examples.

Initially all of these used "high frequency sluggish" germanium transistors, NKT404 in the UK were common, ADZ11 or ADZ12's too. 2N174's in the USA and others. Often though the transformers had moderate leakage inductance and were not ideal, still they were stable and worked.

If you drop a 2N3055 or its pnp equivalent into these Royer circuits they are unstable and oscillate at high frequencies and the transistors cook up. The lively fT needs to be muted down, a 0.1uF base to collector capacitor on each transistor fixes it.

Also, when silicon transistors first started to dominate designs in the late 60's and early 70's for audio, all of a sudden extra parts like 150pF base to collector capacitors started to appear in circuits as it became obvious the simple circuits were unstable at high frequencies due to the spectacular ft of the silicon transistors.

In many ways a lot of early germanium transistor circuits were self protected from troubles at RF because they simply couldn't amplify it. Even a transistor thought of as as "RF" part, like an OC45, has a very large base to collector capacitance (which is why it needs neutralizing in an IF stage) and low range fT compared to the later AF127 or BC557.
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