BJT and FET Frequency response, Lecture notes of Electronics

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BJT & JFET FREQUENCY RESPONSE

FREQUENCY EFFECTS

 AC Amplifiers – the voltage gain decreases when the input frequency is too low or too high  DC Amplifiers – have voltage gain all the way down to zero frequency  Decibels – used to describe the decrease in voltage gain  Bode Plot – used to graph the response of an amplifier

FREQUENCY RESPONSE OF AN AC AMPLIFIER

 Middle Range of Frequencies – voltage gain is maximum / normal operation  Low Frequencies – voltage gain decreases because the coupling and bypass capacitors no longer act like short circuits  High Frequencies – voltage gain decreases due to the internal capacitances across a transistor junctions and due to stray-wiring capacitance

FREQUENCY RESPONSE OF AN AC AMPLIFIER

 Cut-off Frequencies – the frequencies at which the voltage gain equals 0.707 of its maximum values  f 1 and f 2

• lower cut-off frequency and upper cut-off frequency respectively  also referred to as the half-power frequencies

FREQUENCY RESPONSE OF AN AC AMPLIFIER

 Outside the Mid-band  Dominant Capacitor – one that is more important than all others in determining the cut-off frequency

FREQUENCY RESPONSE OF AN AC AMPLIFIER

 Midband:  Below Midband:  Above Midband:

RESPONSE OF A DC AMPLIFIER

 two important characteristics - A V(mid) and f 2  dc amplifier – more widely used than the ac amplifier because most amplifiers are now designed with op-amps instead of with discrete transistors

RESPONSE OF A DC AMPLIFIER

 Between Midband and Cut-off

EXAMPLES

 Example: The figure shows a 741C, an op-amp with a midband voltage gain of 100,000. If f 2 =10Hz, what does the frequency response look like?

DECIBEL POWER GAIN

 Decibels – a useful method for describing frequency response  Review of Logarithms x = 10 y y = log 10 x = log x

• y is the logarithm (or exponent) of 10 that gives x  Using Calculator that has a common log function y = log 10 y = log 0. y = log 100 y = log 0. y = log 1000 y = log 0.  as x increases / decreases by a factor of 10, y increases/decreases by 1

DECIBEL POWER GAIN

 Two Useful Properties of Decibel Power Gain:  Each time the ordinary power gain increases (decreases) by a factor of 2, the decibel power gain increases (decreases) by 3dB.  Each time the ordinary power gain increases (decreases) by a factor of 10, the decibel power gain increases (decreases) by 10 dB. Examples: Calculate the decibel power gain for the following values: Ap = 1, 2, 4, and 8 A p = 1, 0.5, 0.25, and 0. Ap = 1, 10, 100, 1nd 1000 Ap = 1, 0.1, 0.01, and 0.

DECIBEL VOLTAGE GAIN

 Voltage Gain – the output voltage divided by the input voltage A V = v out / v in  Decibel Voltage Gain A V(dB) = 20 log A V example: What is the decibel voltage gain if AV=100,000; 100; 100,000,  Basic Rules for Voltage Gain:  Each time the voltage gain increases(decreases) by a factor of 2, the decibel voltage gain increases(decreases) by 6dB.  Each time the voltage gain increases(decreases) by a factor of 10, the decibel voltage gain increases(decreases) by 20 dB.

DECIBEL VOLTAGE GAIN

 Decibel Voltage Gain A V(dB) = 20 log A V = 20 log (A V ) (A V ) = 20 log A V

• 20 log A V Can be rewritten as: A V(dB) = A V1(dB)
• A V2(dB)

• the total decibel voltage gain of two cascaded stages equals the sum of the individual decibel voltage gains
• additive property of decibel gain example: What is the total voltage gain in the figure? Express this in decibels. Next, calculate the decibel voltage gain of each stage and the total decibel voltage gain.

IMPEDANCE MATCHING

 Concept of Impedance Matching