audioguru said:
Hi Alun,
If the antenna impedance is 75 ohms, the total impedance of the center-tapped coil will be 300 ohms. When saturated, the transistor will have a collector current of about 30mA into the 300 ohms load. If the base bias resistor is 47k, the beta will need to be 170, about right.
Where did you get 300ohms from?
I see what you mean about having about a minimum output from the transmitter of 1.6Vp-p to drive the RF amplifier as 50% class-C. If the transmitter doesn't produce this much, then my RF amplifier will need another RF amplifier, but won't smoke without drive like yours. ;D
1.6V?
This is incorrect.
The transistor has a turn on voltage of about 600mV to 700mV, the sine wave will always be bellow 0V for half the cycle and it will be from 0 to 700mV near zero crossing so the peak voltage will have to be significantly above this for the transistor to conduct for long enough to transmitt a significant amount of power to the load. 50% duty cycle is impossible with this circuit because of the 0 to 700mV points in the crossover part of the cycle prevent it, this is why I suggested a voltage that was bigger than 700mV by a factor of 3.
With an input of 1.6V peak:
sin-1(0.7/1.6) = 26o before it turns on.
This occurs at both crosover points in the 180o positive half of the cycle so:
180o - 64*2 = 128 degrees on per cycle.
This gives a maximum duty cycle of:
128/360 = 35.5%
With 2.1V:
sin-1(0.7/2.1) = 19.5o
180o - 39*2 = 141o
141o/360 = 39.2%
Which I admit isn't much bigger so it woudn't make that much difference but this demonstrates that 50% duty cycle is impossible without biasing the transistor. Also these values are some what optimistic as the transistor won't pass much current when its near its turn on voltage.
4.5V peak would only give 45% maximum.
