Gain of 2N3866 at Lower Voltages
 

Hi All,

Does anyone know what's the power gain of a 2N3866 at 9, 12 and 14
volts? I know that the 2N3866 has a gain of 10dB at 28 volts.

C.W.

On 8 Aug 2005 23:07:14 -0700, wrote:

Hi All,

Does anyone know what's the power gain of a 2N3866 at 9, 12 and 14
volts? I know that the 2N3866 has a gain of 10dB at 28 volts.

C.W.

That must be 1watt out Class C at 400mhz value.

The spec (motorola 1978) Ft is 500mhz @15V and collector current of
50ma. Later manufacture devices have Ft in the range of 800mhz.

The gain is really a function of many parameters and often the given
numbers have to be "qualified" for a given set of test conditions.
The 2n3866 is a high gain device that is useful to in excess of 400mhz
and when used at lower frequencies exhibits the usual Ft/Fop gain
curve( gain at frequency is Ft [where gain is 1] divided by opertaing
frequency and never exceeds the DC or low frequency HFE value).
So at say 40mhz the gain will be around 50 or ~17db.

The gain of that device is tied to frequency and operating current
and circuit impedences more so than voltage. As imagined they are
interrelated. However at 13.6V the class C power gain is around 10db
at 175mhz but at 28V it will be around 13db. In class A or B circuits
the gain numbers have to be derived from the Ft and HFE as well a
circuit topology used. Typically the small signal Class A gain will
be much higher.

Allison
Kb1GMX

As Allison wrote, the gain depends on the operating conditions other
than just collector supply voltage. However, a reasonable rule of
thumb if you don't change the operating class or circuit topology, but
do adjust the output matching network as appropriate, is that the gain
would change by 3dB per doubling of supply voltage (or -3dB for halving
it). Think of it this way: you get the same current, and you're only
changing the voltage. Power equals voltage times current. Of course,
you're limited by the transistor's breakdown voltage, and too much gain
can lead to instabilities. And at very low voltages, you need to
consider the transistor's saturation voltage.

Cheers,
Tom

K7ITM wrote:
As Allison wrote, the gain depends on the operating conditions other
than just collector supply voltage. However, a reasonable rule of
thumb if you don't change the operating class or circuit topology, but
do adjust the output matching network as appropriate, is that the gain
would change by 3dB per doubling of supply voltage (or -3dB for halving
it). Think of it this way: you get the same current, and you're only
changing the voltage. Power equals voltage times current. Of course,
you're limited by the transistor's breakdown voltage, and too much gain
can lead to instabilities. And at very low voltages, you need to
consider the transistor's saturation voltage.

Cheers,
Tom

Tom and Allison,

Thanks for your replies. The reason I'm asking is that I built two
class A small signal VHF stages using 2N3866's, powered by 14 volts,
and I'm getting only 2 to 3 dB gain per stage. Then, I was told to
change the transistors to 2N4427, which are 12 volt devices. Would the
gain improve by changing to 2N4427's? Please explain saturation
voltage. Thanks,

C.W.

On 9 Aug 2005 13:10:53 -0700, wrote:

Tom and Allison,

Thanks for your replies. The reason I'm asking is that I built two
class A small signal VHF stages using 2N3866's, powered by 14 volts,
and I'm getting only 2 to 3 dB gain per stage. Then, I was told to
change the transistors to 2N4427, which are 12 volt devices. Would the
gain improve by changing to 2N4427's? Please explain saturation
voltage. Thanks,

C.W.

Thats horrid performance. I've used 2N3866 at 144mhz and seen far
better gains than that at 12V.

What you don't specify is the (ther than VHF) is the frequency in
use. A transistor with an Ft of 500mhz at 250mhz is going to exhibit
poor gain.

A better choice would be a 2n5179, characterized for lower voltages
and has a hight Ft. I've also used 2n5109 at 440mhz/12V with good
gain, thats what 1200mhz Ft buys you.


FYI: saturation voltage is the votages measured from collector to
ground(usually same potential as emitter) at some specified current
with the device turned on hard (over driven). Applies to switching
circuits usually.

Allison

Many factors influence a stage's power gain other than the intrinsic
gain of the transistor itself. Changes in circuit components and/or
topology are more likely to modify the gain than changing the transistor
type.

Roy Lewallen, W7EL