I need someone to explain this to me plz...the text is long but the questions ar
ID: 1813359 • Letter: I
Question
I need someone to explain this to me plz...the text is long but the questions are simple, i had to write in details to make it as clear as possible.
I have a circuit with an ac input signal, two capacitors, a resistor, a diode , and sn RFC ( radio frequency choke ). The diode is biased at a current of 0.5mA, and an input signal had 100mV amplitude. we need to find the signal level across the diode.
the answer started by finding the ac resistance of the diode as (25mV / 0.5mA), where 25mV is the thermal voltage. I don't care about the rest of the answer, you will see why now.
a second part of the question says: using the same circuit as above, the diode is rated as a 3mA device, and While processing the radio frequency signal, the DC voltage across the didoe is measured as 0.65V. If the input is 100mV find the signal level across the diode.
the answe of this part stated: The diode being a 3mA device, will have a voltage drop of (?) at I = 3mA. The DC voltage of 0.65V will cause a DC current of 0.41mA ( using the equation: V1 - V2 = Vt ln( I1 / I2 ) ).Then the ac resisatnce of teh diode is (25mV / 0.41mA ).
My confusion is the following: In the second part why didnt we find the ac resistance using the same way as in the first part? ( like 25mV / 3mA ). Why did we have to find I2 ? and how was I2 found with that equation, if V1 is 0.65V what was V2 ?
And is there a difference between the words "rated as 3mA" , and "biased at 5mA" ?
Please explain in details, and I wil give 5 stars..this is important.
Explanation / Answer
Bias in a diode is the condition of a diode when voltage across is applied across it.
Or, Biasing in electronics is the method of establishing predetermined voltages and/or currents at various points of a circuit to set an appropriate operating point.
While Diode rating is nothing but is similar to the peak inverse voltage/current
which is : If this rating is exceeded the diode may be destroyed
V2 is the voltage applied ie., 25mV