Ft of the transistor

[walid1]

Hi

Ft is the freq at which beta (hfe) = approx 1
for 2N3904 transistor I found that at 300MHz, hfe = 1

frome the same datasheets i found the following curve:

look at the red curve
at 300MHz, hfe = 5 dB
20 log (hfe) = 5
hfe = 1.8

Now at FM freq (say 100 MHz) and from the red curve, hfe = 15dB
that is hfe = 5.6 (very low)

In the past, when I was looking for a FT, and I find it = 300MHz, I think that hfe would be a big
value at FM freq = 100 MHz but it seems that the curve will be worth only about 6
My question: Is this enough when using this transistors as oscillator at 100MHz?
gURU use that transistor in his FM TX.
thanks

 

[audioguru2]

Hi Walid,
My FM oscillator uses the transistor's alpha, because it is a common-base amplifier. A common-emitter amplifier needs to have some beta.

An oscillator needs to have a minimum voltage gain of slightly more than only 1.

 

[walid1]

My FM oscillator uses the transistor's alpha, because it is a common-base amplifier.

(1)I know that it is a common-base amplifier because of C5 connected between base and GND and at FM freq it short that base to GND.
But I did not know about the transistor's alpha, can u please tell me something about it?

(2)I attach your FM Tx,

I look at Q3, it is a 2N3904 transistor and it deal with 100MHz signal to amplify it.
How it can do this with only hfe =6 at 100MHz?
thank you guru, I wish that I'm not disturbing you by these frequently questions.

 

[audioguru2]

I simulated my FM transmitter and increased the value of its oscillator's feedback capacitor. It produces 11V p-p across a 75 ohms load which is 202mW RMS.

The collector current has a peak of 95mA. The transistor's collector current drops to zero on each cycle. As I suspected it operates in class-C.
The base has positive and negative 20mA peaks.
So the Hfe is 95/20= 4.75.

 

[Kevin Weddle]

An hfe = 6 makes for a real low input impedance. Since the collector current is a fraction of the emitter current, the output signal size is less. If your VCE isn't high like the 20V shown in the data sheet, your hfe will be even lower. Also, the more you change the hfe by a high change in collector current, the less linear the reproduction.

Audioguru, with a tank at the collector of a transistor, the collector averages out at the supply voltage? So a DC supply of 9v might go 10v to 8v?

 

[audioguru2]

An hfe = 6 makes for a real low input impedance.

It doesn't matter because the emitter of the oscillator can supply enough base current to the output transistor.

If your VCE isn't high like the 20V shown in the data sheet, your hfe will be even lower.

Good point, Kevin. They measure the ft with a collector/emitter voltage of 20V and measure the high frequency hfe with 40V to reduce the transistor's capacitance so it has better results at the high frequencies. But there is enough hfe for this circuit.

Also, the more you change the hfe by a high change in collector current, the less linear the reproduction.

It doesn't matter because the tuned parallel LC circuits attenuate the harmonics.

Audioguru, with a tank at the collector of a transistor, the collector averages out at the supply voltage? So a DC supply of 9v might go 10v to 8v?

A very high voltage swing can occur across a tuned parallel LC circuit. This one could easily exceed 30V p-p if the output transistor had more current or if the circuit had no load. That is why transmitters warn you not to use them if their antenna is not fully extended. The high voltage across the output transistor would destroy it.

I simulated my last circuit without the 75 ohm antenna load. The collector swung up to +20V.

 

[AN920]

One should always remember that SPICE type simulators  like the LT assume perfect components. In real life the Q of coils and capacitors at the operating frequency makes a big difference. A more accurate simulation will be with a simulator that use harmonic balance techniques and simulate transistor operation under non-linear conditions using non-linear models. These simulators also use very accurate models for the passive components.

SPICE simulators are intended for time domain and not suited for frequency domain analysis.

 

[walid1]

Hi
I read carefully Kevin Weddle's reply an Audioguru's reply, but sorry i can't understand anything, i don't know why. Also I can't ask questions because i don't know about what to ask!
Despite that will ask :

I simulated my last circuit without the 75 ohm antenna load. The collector swung up to +20V.

how 20v and the supply voltage is only9v????

 

[audioguru2]

Hi Walid,
Think about a child on a swing. A small push at the correct moment makes it swing higher and higher. It can easily swing higher than the bar that holds it. If you keep adding small pushes then it goes higher and higher.

The bar of the swing is the supply voltage. The swing resonates back and forth like a parallel tuned circuit passes current back and forth between the coil and the capacitor. The swing and the output voltage go higher.

 

[Kevin Weddle]

It doesn't matter because the tuned parallel LC circuits attenuate the harmonics.

A PLL has to control the output of a VCO to make a good sinewave. If you don't control a parallel LC circuit, how can it make a good sinewave regardless of how the transistor is operated? I would think a transistor operated most linearly would make a better sinewave.

 

[audioguru2]

Kevin,
In a PLL, the carrier can be a sine-wave which is made by the VCO. The VCO's frequency is modulated.

We are talking about the LC parallel tuned circuit reducing the harmonics of the 100MHz carrier frequency, not the modulating audio frequencies.
The RF harmonics are AM but we are modulating the frequency of the carrier to make FM.
The parts that are voltage-controlled capacitors are very linear over the small amount of capacitance change that is needed to make FM modulation.

These toy FM transmitters have low power so their RF harmonics don't cause much interference to TV signals and other communications.

 

[Kevin Weddle]

I was talking about the oscillator itself. If we were to take away the modulation, the oscillator would be of a certain quality. Your saying the LC produces a good oscillator regard less of the transistor operation. The change in collector current looks excessive. But your saying it has no affect on the sinewave produced.

 

[audioguru2]

Kevin,
The RF amplifier in many transmitters operate in class-C for good efficiency. A transistor operating in class-C is completely turned off for some of the time of each cycle, but the high Q of the tuned circuit attenuates the harmonics so the output is a pretty good sine-wave.

 

[Kevin Weddle]

Class C used for power audio creates an abridged version of the audio signal. And that means you lose a lot of what was in the audio signal.

The oscillator looks like it has 90 degrees feedback. Have you tested this transmitter with a simple receiver? How does it sound?

 

[audioguru2]

Class C used for power audio creates an abridged version of the audio signal. And that means you lose a lot of what was in the audio signal.

I don't think class-C is ever used for audio. It would be far too distorted.
Class-D amplifiers are very efficient because they are a switching device and their distortion is pretty low now.

The oscillator looks like it has 90 degrees feedback. Have you tested this transmitter with a simple receiver? How does it sound?

My FM transmitter has an oscillator with its collector output at the same phase as its emitter input.
The circuit has a separate RF amplifier to keep things near its antenna from changing its tuned frequency. The RF amplifier also increases its range.
It has a low-dropout voltage regulator to keep the rf frequency from drifting as the battery voltage runs down and to keep the preamp transistor from distorting.
It has pre-emphasis which is treble boost like FM radio stations have so it sounds crisp and clear. It sounds perfect except it is mono, not stereo.

 

[Kevin Weddle]

quote]
My FM transmitter has an oscillator with its collector output at the same phase as its emitter input.

The signal at the emitter of the oscillator leads the collector signal by 90 degrees because of C7. That means the emitter lags the base by 90 degrees.

I also understand that simple oscillators are just that, simple. They are not signal amplifiers.

 

[audioguru2]

The signal at the emitter of the oscillator leads the collector signal by 90 degrees because of C7. That means the emitter lags the base by 90 degrees.

No. C7 is in parallel with the 2pF of the transistor plus 5pF for stray wiring capacitance. The total of 11.7pF has a reactance the same as the emitter resistor at about 62MHz where the phase shift is 45 degrees. At 100MHz the phase shifrt is less, maybe 25 or 30 degrees.

The base doesn't have an RF signal because it is AC grounded by C5. Its 470pF has a reactance of only 3.4 ohms at 100MHz. The base is used for audio frequency modulation.

Someone on another website showed that if C7 is increased to 10pF then the output power is higher.

I also understand that simple oscillators are just that, simple. They are not signal amplifiers.

No. The oscillator is a common-base signal amplifier with positive feedback added to make it oscillate.

 

[Kevin Weddle]

I see. The emitter resistor has little affect on the amount of feedback or phase shift. C5 divides by beta making the feedback almost totally capacitive. That means the collector signal is in phase with the emitter signal. Too strong a base signal would put the base signal in phase with the emitter signal and then the emitter signal would be out of phase with the collector signal. And so on...

It's interesting that since all the positive feedback reaches the emitter base junction, the gain produced by the LC is not affected, otherwise the LC would be lowered by the parallel resistance.

 

[audioguru2]

That means the collector signal is in phase with the emitter signal.

Yes because the transistor is a common base amplifier.

Too strong a base signal would put the base signal in phase with the emitter signal and then the emitter signal would be out of phase with the collector signal.

There is no RF base signal, the base is shorted to ground at RF by C5.