ENAE 362

The circuit in the 555 is referred to as what (2 answers)?

"one shot" or monostable

What is the cutoff frequency often specified as?

-3dB point; or responds to power ratio of 1/2; also when Eout = .707Ein

The low from the flip flop in the 555 does two things. What are they?

1) A low applied to the base of the discharge transistor essentially disconnects pin 7 from ground. This allows the 500 mfd capacitor to start charing through the 100K pot (555-2-3). 2) The inverting output stage causes pin 3 to go high, hence the LED comes on

A typical capacitor voltage divider circuit is seen on lecture notes AC-14 (Ein connected to two capacitors in series; The voltmeter for Eout is placed in between C1 and C2). 1) What does Eout=? 2) Does frequency matter in the voltage division ratio? 3) Is Eout in phase with Ein?

1) Eout = [Xc2/ (Xc1+Xc2)]*Ein = [C1/(C1+C2)]*Ein 2) The voltage division ratio is independent of the frequency even though each capacitor's reactance is dependent on the frequency 3) Yes

Look at the circuit on lecture notes pg S-30. Answer the following questions: 1) SCR1 controls what through Rl? 2) What is/are used to turn off SCR1? 3) What turns on SCR1 and what happens when it is on? 4) What happens when a short current pulse is applied to SCR2? 5) What causes SCR1 to become reversed biased?

1) Power 2) C, R1, and SCR1 3) A short current pulse; allows large dc current to flow through Rl and the capacitor also charges up 4) SCR2 turns on and effectively connects the capacitor across SCR1. 5) The large reverse voltage when the capacitor is connected across SCR1. The current flow then drops below the holding current Ih and the device turns off.

The high from the flip flop in the 555 does two things. What are they?

1) The high applied to the base of the discharge transistor turns on the transistor; the 500 mfd capacitor very quickly discharges to ground through pin 7 (555-2-3). 2) The inverted output on pin 3 goes low, hence the LED goes out.

Ib is typically what percent of Ie? Ic is...

1-5%, 95-99%; Note Vb is always about .7 Volts

Xc (Capacitive Reactance) = ...

1/(2*pi*f*C)

Series capacitor formula...

1/Ct = 1/C1 + 1/C2

Parallel inductor formula...

1/Lt = 1/L1 + 1/L2

Which two pins are referred to as the "inputs" in the 555?

2 and 6

What is the voltage at the inverting (-) input to op amp 1 and the non inverting (+) input to op amp 2 in the 555?

2/3Vcc; 1/3Vcc

What is a half wave rectifier?

A circuit that only allows half of the ac sine wave to pass through (See lecture note S-11 for a picture)

What is a current steering diode?

A current steering diode blocks current, so it steers it.

What is a diac?

A diac is a bidirectional trigger diode. The diac acts like two PN junction diodes connected back to back. The disc turns on in either direction when the voltage exceeds the breakdown voltage of the PN junction that is reversed biased. The diac functions like a bidirectional switch which turns on when the breakdown voltage (in either direction) is exceeded.

What does a diode always need?

A traveling companion, which is a resistor. Otherwise the diode will burn out

What is a triac?

A triac is a bidirectional triode thyristor. Unlike the SCR which is a unidirectional device, can conduct current in either direction. Note: the trigger voltage for current to conduct is different for both directions. Also note: trials typically are used to control ac power

What is the gain of an open loop op-amp? Is it stable?

About 200,000; very unstable

What is a bypass capacitor?

Allows ac component of a signal to be shunted or bypassed around Re.

On pg OA-5, what is the circuit called? What must Eout be?

Analog adder (could also be turned into subtractor, integrator, and differentiator); -5V to allow current flow (A must remain a virtual ground); Note: A is also called the summing point.

What is closed loop mode?

Closed loop mode involve taking part of the output and feeding it back into one of the inputs.

What is an op amp called when it is in open loop mode?

Comparator because it compares the two input levels and responds with the appropriate output level (- > + output is low; + > - output is high)

Parallel capacitor formula...

Ct = C1 + C2

What are the two modes of a IGFET/MOSFET?

Depletion and enhanced (both with N and P channel types)

How would you derive the cutoff frequency?

Derive the equation from Ein^2 = Eout^2 + Er^2; An easier approach is to say Xl = Xc; (2*pi*f*L) = 1/(2*pi*f*C); fc = 1/(2*pi*sqrt(LC)) = 1/2*pi*R*C

tan(theta) (phase shift angle) =

Ec/Er = Xc/R

What is inductance?

Electrical phenomenon involving magnetic field lines that oppose a change in current flowing through a conductor.

What does Eout for a subtractor circuit equal?

Eout = (R1/R2) * (E1-E2)

What is forward bias?

Forward bias allows for current to flow once a certain barrier voltage has been reached.

P (Power) = ...

IE

The emitter current (Ie) equals

Ic+Ib (collector plus base)

When a zener diode is being used as a voltage regulator, what happens when Rl changes and Ein is held constant?

If Rl increases, Irl will decrease, and so the total current also decreases. This means that the voltage across the R's would go down. V and I would then increases across the zener.

When a zener diode is being used as a voltage regulator, what happens when Ein changes and Rl is held constant?

If Rl increases, the voltage will also try to increase across the resistor which means there will be less current flow. The voltage across the zener tries to remain constant.

What will happen if the LED is connected to the Vcc (pin 8) in the 555?

If the pin 3 goes low, the LED will light up. The 555 is said to be "sinking" current in this case (conventional current).

What is impedance?

Impedance is the total opposition to current flow in an ac circuit. Z = sqrt[R^2 + (Xl-Xc)^2]

What does (-) mean on the op amp? (+)?

Inverting input; noninverting input

What does a one shot circuit mean?

It has one stable state

Series inductor formula...

Lt = L1 + L2

What is mesa type?

Mesa type construction is sometimes used in reference to the epitaxial growth transistors.

In a series RC circuit, if C is small...

Most of the voltage drop will occur across R

What are the two types of JFETS?

N channel and P channel

What are two types of bipolar transistors?

NPN, PNP

How can one solve the thermal drift problem of based biased circuits?

One method is where a small portion of the output current is fed back to the input to control the tendency of the circuit to drift with temperature. The circuit on A-4 has a resistor connected between the base and collector. This type of temperature compensation is referred to as degenerative or negative feedback.

After a certain amount of time in the 555, the top of the 500 mfd reaches 2/3Vcc (555-2-3). What happens when the (+) input to op amp 1 (pin 6) goes higher than 2/3Vcc (pin 5)?

Op amp 1 gives out a high voltage level that causes the flip flop to "reset" and the output from the flip flop goes high.

What are operational amplifiers? What are differential amplifiers?

Op amps are differential amplifiers on a chip. Differential amplifiers are amplifiers that amplify the difference between two input signals.

What are the two operational modes of an amplifier?

Open loop and closed loop

What is the power factor (PF)?

PF = true power/apparent power

What is the quality of inductors?

Q = Xl/R; Note: The perfect inductor has only inductance. However in reality, an inductor has inductance and reactance. The quality of the inductor is given by Q. A good inductor has a quality greater than 20.

With resonance, Z = ...

R

Normally, how are the resistances in a subtractor chosen (See lecture notes OA-11)?

R1/R2 = R3/R4

For an inverting amplifier (like on OA-8) what is the value of R3? This is also the same for a non inverting amplifier.

R3 = (R1*R2)/(R1+R2)

The circuit on the bottom of lecture notes A-4 is said to have emitter feedback. What does this mean?

Re is connected between the emitter and the ground. This results in Vcc being applied across Rb and Re. When T increases, Ic and Ie will too, Ere will increase, so Erb will have to decrease which leads to a decrease in Ib which then leads to Ic and Ie to return to the normal bias levels.

What is the most common use for diodes?

Rectification

What effect does closed loop mode have?

Reducing gain (from about 200,000 to 20 for example, and allows for a stable linear operation

What is voltage divider biasing?

Replace Rb in the emitter feedback with two resistors (see lecture notes A-5).

When the 741 op amp is in open loop mode, how is it connected?

See lecture notes OA-4

Which came first: solid state or cathode tubes?

Solid state

What is a difference amplifier also known as?

Subtractor

For depletion mode device JFET, what does Vgs = 0 mean?

The JFET will conduct a large drain current when Vgs = 0; we say these devices are normally on.

What is the depletion region?

The absence of free electrons and free holes in a small zone to either side of the PN junction.

What defines a based biased circuit?

The base current is controlled by Vcc and Rb.

What are the unique characteristics of the PN junction diode due to?

The behavior of the electron hole pairs in the junction or interface region

The based biased circuit is thermally unstable. What does this mean?

The bias currents (Ib and Ic) tend to change with temperature

In a 555 timer, what makes the timing process?

The capacitor and resistor form the timing process

What is the potential barrier or barrier voltage?

The charge that develops across the PN junction creates a difference in potential or voltage. This is what the difference is known as.

What is the voltage at pin 5 referred to as in the 555?

The control voltage; 2/3Vcc from pin 1 is available here

What is one main difference between a 555 kitchen timer and the deer away circuit (which consists of a 555)?

The deer away circuit is continuously running.

For enhancement mode IGFET/MOSFET, what does it mean when Vgs = 0?

The device is aid to be normally off; no current can flow between the source and the drain

What is reversed bias?

The diode develops a very large barrier voltage because the depletion region gets larger. This effectively means the diode stops conducting.

What happens at the breakdown voltage? Note: the reverse bias current is very small until this point is reached

The diode starts to conduct a very large current in the reverse direction. The voltage which causes this to happen is the breakdown voltage

What happens when the voltage level to the reset input goes high on the flip flop in the 555?

The flip flop is "reset" and the output goes high

What happens when the voltage level to the set input goes high in the 555?

The flip flop is "set" and its output goes low

What is breakdown frequency?

The frequency where the breakover point occurs.

What happens to the gain at breakdown frequency?

The gain drops linearly to 1 or unity gain. The frequency once it has reached unity gain is the unity gain frequency.

The output from the flip flop is also connected to the base of the discharge transistor. What happens when the flip flop is reset in the 555? What about when the flip flop is set?

The high applied to the base of the transistor causes pin 7 to be shorted to ground (pin 1); The low applied to the base of the transistor causes the transistor not to conduct, so pin 7 appears to be open.

In the 555, what happens when the switch to pin 2 is temporarily closed?

The inverting (-) input to op amp 2 goes to ground, which means it goes below the non-inverting (+) input that is at 1/3Vcc. Op amp 2 then gives out a high voltage level that causes the flip flop to "set" and the output of the flip flop goes low.

What happens to the output from the flip flop in the 555?

The output is amplified and is inverted by the inverting output stage, which is then available at pin 3. If the input is high, the output will be low and vice versa.

What is the output of pin 3 connected to in the 555?

The output is connected to a "load" such as an LED. Note: If the other end of the LED is connected to ground, the LED will light when pin 3 is high. The 555 is said to be "sourcing" current (conventional current).

What is the breakover point?

The point where the signal has dropped to 70.7% of the 0Hz or dc level.

What is rectification?

The process of converting ac to dc

What is an inductor?

The typical inductor is a concentrated winding or coil or electric wire; for this reason they are many times referred to as coils. Note: Every conductor is a weak inductor.

On pg S-13 in the lecture notes, there is a circuit. Describe what is going on.

The zener diode will keep the voltage practically constant, while the resistor Rs will burn off the excess voltage

Where are the outputs of pins 2 and 6 fed to in a 555?

They are fed to a device known as the flip flop. Note: the outputs of pins 2 and 6 will either be high or low depending on the inputs

What is the purpose of the two op amps in the 555?

They essentially form a dual comparator circuit

On pg OA-4, what is point A referred to as?

Virtual ground because the voltage at point A relative to ground is 0.

What does FET mean with transistors?

Voltage controlled

Vcc=... (2 answers)

Vrl + Vce and Vbe+Vrb

What does it mean when a circuit uses collector feedback?

When T increases, Ic and Erl will increase while Vce will decrease which decreases V across Rb. Ib decreases which causes a decrease in Ic. This type of feedback does not provide complete temperature stability.

In the circuit on lecture notes A-3, what happens when temperature increases?

When T increases, the total current (Ic+Ico where Ico is the leakage current), and leakage current goes up. This results in the voltage drop across R to increase.

What is self-induction?

When the flow of an electrical current is made to increase in a conductor, a counter or back EMF is produced in the wire that oppose the increasing current. This is caused by the relative motion between the expanding (and strengthening) magnetic field and the conductor. The opposite occurs when the current is made to decrease.

What is the main difference between AC and DC voltage?

With DC, current always flow in one direction. With AC, current can flow in both directions

What is inductive reactance (Xl)?

Xl = 2*pi*f*L

Is the output (Eout) of a non inverting amplifier in phase with Ein?

Yes

What is the current gain for a common emitter transistor?

beta = delta (Ic) / delta (Ib) at some Vce; for the transistors we're using, it's 100 to 1

Amplifiers that use a transistor are classified as...

common-emitter, common-collector, or common-base

What are three methods to produce the PN junction wafer?

diffusion, grown, and alloyed (more information lecture notes S-5)

For a low pass filter, the order from (left to right) of the inductor and resistor is...

inductor, resistor (L, R)

Semiconductors are commonly referred to as...

rectifiers

For a high pass filter, the order from (left to right) of the inductor and resistor is...

resistor, inductor (R, L)

Amplifiers can be...

voltage or power amplifiers