BJT Amplifier Design

Emitter Configuration

• In a common collector(CC) stage, also known as emitter follower, the emitter is at AC ground.
• In a common emitter(CE) stage, the output voltage is typically dependent on re when an emitter-bypass is incorporated.
• Voltage gain is the output voltage divided by the input voltage.

Input Impedance

• The input impedance of the base decreases as Beta (β) increases.

Voltage Gain and Resistance

• Voltage gain is directly proportional to the AC collector resistance.

Swamped Amplifier Characteristics

• The feedback resistance of a swamped amplifier is large compared to the AC resistance of the emitter diode.
• Compared to a common-emitter (CE) stage, a swamped amplifier features a larger input impedance.
• Increasing the emitter feedback resistance reduces the distortion.
• The emitter of a swamped amplifier has an AC voltage.
• A swamped amplifier employs negative feedback in its design.
• The impact of the emitter diode becomes unimportant in a swamped amplifier.
• The feedback resistor reduces distortion in the amplified signal.
• The feedback resistor stabilizes voltage gain.

AC Collector Resistance

• The AC collector resistance of the first stage includes the input impedance of the second stage.

Effects of Component Failure

• If the emitter-bypass capacitor opens, the AC output voltage decreases.
• If the emitter-bypass capacitor shorts, the DC base voltage increases.
• If the collector resistor shorts, the AC output voltage decreases.
• If the load resistance opens, the AC output voltage increases.
• If any capacitor opens, the AC output voltage decreases.
• If the input-coupling capacitor opens, the AC input voltage decreases.
• If the bypass capacitor opens, the AC input voltage at the base remains the same.
• If the output-coupling capacitor opens, the AC input voltage remains the same.
• If the emitter resistor opens, the AC input voltage at the base remains the same.
• If the collector resistor opens, the AC input voltage at the base is approximately zero.
• If the emitter-bypass capacitor shorts, the AC input voltage at the base is equal to zero.

Multistage Amplifier Effects

• If the input impedance of the second stage decreases, the voltage gain of the first stage decreases.
• If the BE diode of the second stage opens, the voltage gain of the first stage remains the same.
• If the load resistance of the second stage opens, the voltage gain of the first stage increases.

JFET Basics

• A JFET is voltage controlled
• A unipolar transistor uses either free electrons or holes.
• The input impedance of a JFET approaches infinity.
• The gate controls the width of the channel, drain current, and gate voltage.
• The gate-source diode of a JFET should be reverse biased.
• Compared to a bipolar junction transistor (BJT), a JFET has a much higher input resistance.
• Pinch-off voltage magnitude is equal to the gate-source cutoff voltage.
• A JFET acts like a current source when the drain saturation current is less than IDSS.
• RDS equals pinch-off voltage divided by the drain current for zero gate voltage.
• Transconductance curve is nonlinear.
• The transconductance increases when the drain current approaches ID(sat).
• A CS amplifier has a voltage gain of gmrd.
• A source follower has a voltage gain of gmrs/(1+gmrs)
• A source follower has some distortion when the input signal is large.
• A source follower has a voltage gain of less than 1, some distortion and high input resistance when the input signal is large
• The input signal used with a JFET analog switch should be a square wave.
• A cascode amplifier has the advantage of large voltage gain and low input capacitance.
• VHF(Very High Frequency) covers frequencies from 30 to 300 MHz.
• When a JFET is cut off, the depletion layers are touching.
• In an n-channel JFET, the channel between the depletion layers shrinks.
• If a JFET has IDSS = 8 mA and VP = 4 V, then RDS equals 500 Ω.
• The easiest way to bias a JFET in the ohmic region is with self-bias.
• Self-bias produces negative feedback.
• A negative gate-source voltage in a self-biased JFET circuit requires a source resistor.
• Transconductance is measured in mhos or siemens.

Transconductance

• Transconductance indicates how well the input voltage controls the output current.