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Purpose of rating 120MHz to the "audio" transistor?

Willen

Well-Known Member
Hi, As an example, I have upload a datasheet of 2sc2328 audio purpose transistor. The datasheet clearly mentioned it as Audio Power Amplifier transistor. But in detail the transient frequency or current gain band product has "120MHz at 2V, 500mA". If the transistor is designed to drive audio the why they are playing around 120MHz? And when I ask any expert to use it around HF and VHF band, they directly reject! Why they do not test the different parameters of transistor around audio frequency? But testing the transistor to VHF range instead?
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
Because that's what the rating of the transistor is - proper RF transistors will usually be higher, and specifically designed for RF use - a different configuration inside (capacitance etc.)

Low power audio/switching transistors are very often rated at 300MHz, and easily work as RF 'bug' type transistors at VHF frequencies.
 

danadak

Well-Known Member
Most Helpful Member
Also a function of the G used in the amplifier for that specific stage
of the amp.

If one wants G = 100 @ 20 Khz then the GBW has to exceed 100 x 20 Khz
= 2 Mhz. That being said hard to image one wanting 120 Mhz / 20 Kz =
~ 6000 = G which would be ridiculous. Stated otherwise the 120 Mhz is
not needed.....In fact that wide a BW means care has to be taken in circuit
design transistor does not oscillate at RF as all an OSC needs is to overcome
a few losses, so does not need much G to oscillate.


Regards, Dana.
 

JimB

Super Moderator
Most Helpful Member
But in detail the transient frequency or current gain band product has "120MHz at 2V, 500mA"

Let us clarify...
The "gain-bandwidth product" is a number produced by multiplying the voltage gain of the transistor by the frequency at which you are measuring the gain,

The gain of the 2SC2328A is classified onto two groups Rank O and Rank Y, as per the little box at the bottom of the datasheet.

Say we have a transistor with a gain of 100.
That is the gain at low frequencies.

If we divide the 120MHz GBP by 100 we get 1.2MHz
So we have a gain of 100 up to 1.2MHz.
At higher frequencies the gain will drop off until at 120MHz the gain will be 1.

A quick and dirty graph of Gain vs Frequency will look like this:

1666798485472.png



It is quite common to use "audio" transistors like this at low RF frequencies.

JimB
 

crutschow

Well-Known Member
Most Helpful Member
Why they do not test the different parameters of transistor around audio frequency?
They test if for the frequency response it is capable of, not the frequency you may use it at.
That way you can properly design the circuit for your requirements, be it audio, video, RF, or other.
That's always the way such devices are specified.
 

danadak

Well-Known Member
Most Helpful Member
One might see distortion, linearity tests, at audio freqs, but GBW is
needed to know if you can achieve the BW at the G you have as a
goal.

Of course many tests at DC to insure the component can withstand
damage if those limits are exceeded.


Regards, Dana.
 

ronsimpson

Well-Known Member
Most Helpful Member
The speed rating of a transistor is takes at the "sweet spot". At more current it probably is slower and at less current it is defiantly slower. If the voltage across the part, C-E, is too small the part gets slow.
 

Willen

Well-Known Member
Hi again, Another interesting 'not rated rating' is RF power output (P in and P out) of some RF purpose transistor like 2sc3357 (SMD). The transistor has great transient frequency of around 6GHz and has 1.2 watts power dissipation and 100mA collector current. Maybe it means it can push more RF power even in UHF than general BJT. But all of the datasheets do not have RF output rating. They just say it as "RF transistor".
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Maybe it means it can push more RF power even in UHF than general BJT. But all of the datasheets do not have RF output rating.

As an example, a 1A 500V 0.1 Ohm MOSFET rated at 1W dissipation could hypothetically control a load rated at anything up to 500W, using hard switching.

Use it in a linear mode and the power out capability would be minuscule by comparison of that.

It's all down to how a device is used in a particular circuit; Class A dissipates most power (compared to output capability) down to Class C or D, which dissipate least power (relative to output power).

If a device has an output power rating, it's likely given for a specific circuit or class of operation.

Class C is common in many transmitters and frequency multiplier stages, but it must have proper tuned circuits or low pass filtering after it to eliminate the vast range of harmonics the unfiltered signal would contain.
 

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