Semiconductors2 - U3 - Op Amps
How do you calculate gain on an inverting amplifier?
-(R(f) / R(i)), where R(f) is the resistor on the feedback loop, and R(i) is the resistor input resistor going to the (-) non-inverting input.
If you are troubleshooting an inverting amplifier with input voltage of 100 mV, what voltage should you find on the inverting input?
0 V
How do you calculate the gain for a non-inverting input?
1 + (R(f) / R(i)) where R(f) is the resistor on the feedback loop, and R(i) is the resistor input resistor going to the (-) non-inverting input.
If you are troubleshooting a non-inverting amplifier with input voltage of 100 mV, what voltage should you find on the inverting input?
100 mV
What is a buffer?
A buffer is a device that passes a signal between two circuits while preventing the load circuit from loading down or adversely affecting the source circuit.
What is an averaging amplifier?
A variation on a summing amplifier that produces the negative value of the mathematical average of the input voltages.
How is the attenuation (B) of a non-inverting amplifier related to the closed loop gain (A(cl(NI)))?
A(cl(NI)) = 1 / B
Why are buffers are used to couple circuits.
Buffers are used when the source signal is too weak to drive the load. Because of the op-amps high input impedance and low current draw it has minimal effect on source circuit while boosting output to the load.
How is CMRR calculated in decibels (dB)?
CMRR in (dB) = 20 log(CMRR)
What is the common mode rejection ratio (CMRR)?
CMRR is the measure of an amplifiers to reject common mode signals. The formula is: CMRR = Differential gain (A(v(d))) OR open-loop gain (V(ol)) / common-mode gain (A(cm))
Name the 3 types of amplifier circuits that make up an op-amp.
Differential amplifier, voltage amplifier, push-pull amplifier.
(True or false) The closed-loop voltage gain of each of the op-amp configurations (inverting and non-inverting) is dependent on the internal open-loop voltage gain of the op-amp.
False
What is a virtual ground? What circuit is it found in?
Found in an inverting amplifier where there is no voltage drop between the inverting (-) and non-inverting (+) inputs. This means that the voltage at the inverting input terminal is zero because the non-inverting input is grounded.
What is the schematic symbol of a differential amplifier?
Has two inputs and two outputs. Refer to SDR2.46
What are 3 characteristics of the ideal op-amp model?
Ideal op-amp has infinite voltage gain, infinite input resistance (or input impedance), and zero output resistance (or output impedance).
What do the outputs look like if AC is connected in a single-ended input in a diff-amp?
If input 1 is connected to AC and input 2 is grounded the output 1 will appear as an amplified, inverted voltage signal of input 1, while output 2 will appear as an amplified signal of input 1 (Refer to top diagram SDR2.47). If input 2 is connected to AC while input 1 is grounded, output 2 is amplified inversion of input 2, and output 1 is amplified of input 2 (Refer to bottom of SDR2.47).
If a comparator has two inputs (see SDR2.55) how is it determined what the output would be?
If the non-inverting voltage (+) is higher than the inverting voltage (-), then the output would be maximum positive. If the inverting voltage (-) is higher than the non-inverting voltage (+), then the output voltage would be maximum negative.
How does a diff-amp operate in differential or double-ended mode?
In this mode two signals of opposite polarity are applied to the inputs. In top right of SDR2.48 shows the output if this signal in input one is considered. Bottom left of SDR2.48 shows the output if only the signal at input 2 is considered. Note that the outputs from inputs 1 and 2 are the same. When combined (SDR2.48 bottom right) they result in the same waveforms with twice the amplitude.
Define input offset current (I(OS))
Is the difference of input bias currents expressed as an absolute value: I(OS) = |I(1) - I(2)|
Define input offset voltage (V(OS)).
Is the differential dc voltage required between the inputs to force the differential output to zero volts.
Define bias current I(BIAS).
Is the direct current required by the inputs of the amplifier to properly operate the first stage. By definition, the input bias current is the average of both input currents: I(BIAS) = (I(1) + I(2)) / 2
Define open-loop voltage gain (V(ol))
Is the internal voltage gain of the device and represents the ratio of output voltage to input voltage when there are no external components.
What is a differential amplifier?
It is an amplifier that produces an output proportional to the difference of two inputs.
What is a comparator?
It is an op-amp circuit that can determine if an input voltage exceeds a certain level.
What is a common mode condition in diff-amps?
It is any ac or dc signal that is identical on both inputs. Refer to SDR2.49. Top right refers to input 1 only, bottom left refers to input 2 only. The outputs from inputs 1 and 2 are inverse of one another. When combined they cancel each other out resulting in near zero voltage (bottom right SDR2.49). This is called common mode rejection.
Define Common Mode Input Voltage Range
It is the range of input voltages which, when applied to both inputs, will not cause clipping or other input distortion. Many op-amps have common mode input voltages from -10 to 10 volts with dc supply of -15 to 15 volts.
What is a single ended input?
It is when a differential amplifier (diff-amp) has one input grounded and signal voltage is applied to the other circuit.
How is an averaging amplifier constructed?
Must arrange resistances so that the gain, R(f) / R(i), is the reciprocal of the number of inputs. E.g. if there are 4 inputs, R(f) / R(i) must equal ¼, if 3 inputs must equal 1/3, ect. See SDR2.59.
What is negative feedback?
Negative feedback is a process by which a portion of the output voltage of an amplifier is returned to the input with a phase angle that opposes or subtracts from the signal.
What are common mode signals?
Noise or interference generally as a result of picking up radiated energy on input lines, from adjacent lines, or 60 Hz power line, or other sources.
What is the schematic symbol of an operational amplifier (op-amps)?
See SDR2.43
Draw or describe the structure of an inverting amplifier.
See SDR2.50. Voltage input and feedback loop goes to the (-) inverting input, and ground goes to non-inverting input (+).
Draw or describe the structure of a non-inverting amplifier.
See SDR2.51. In this circuit the voltage input is connected to the (+) non-inverting input, ground and negative feedback loop are connected to (-) inverting input.
What does a comparator circuit look like?
See SDR2.52. Has an op-amp with two inputs and no negative feedback loop.
How do you construct a comparator that DOES NOT invert the input?
See SDR2.53. Attach ground to inverting terminal (-) attach voltage input to non-inverting input (+).
How do you construct a comparator that inverts the input?
See SDR2.54. Attach ground to non-inverting input (+) and input voltage to inverting input (-).
How is a summing circuit built?
See SDR2.57. Is an op-amp with negative feedback to the inverting (-) input. Non-inverting input (+) is ground. Voltage inputs to be summed are connected to the inverting input (-).
What is the formula for calculating the result of a summing circuit when all input resistors (R(i)) are all the same?
See SDR2.58
What is the formula for calculating the result of a summing circuit when input resistors R(i) have different resistances like SDR2.60?
See SDR2.61
How are buffer circuits constructed?
See SDR2.62. Consists of an op-amp with a negative feedback loop connected to the inverting input (-). Source signal and ground connected to the non-inverting input (+). Load attached to op-amp's output.
Why is negative feedback used?
See lab 1 part A. Without negative feedback op-amp output would always go towards saturation for small changes in voltage (either max positive or max negative). With negative feedback op-amp output can be more incremental.
Define summing point.
Summing point is the terminal of the op-amp where the input resistors are commonly connected.
What are two specifications that are important for comparators?
Switching speed and bias current.
Which input on the op-amp is inverting and which is non-inverting?
The (+) input is non-inverting, and the (-) is inverting
Define slew rate (SR). What are the units used.
The maximum rate of change of the output voltage in response to a step input voltage. Expressed as change in V(out) / change in time where V(out) is +V(max) - (- V(max)). The units are volts per microsecond.
Define output resistance (R(out)
The resistance viewed from the output terminal.
Define input resistance (R(in))
Two ways of specifying input resistance of op-amp: differential input resistance (R(in(diff))), total resistance between the inverting and non-inverting inputs; common mode input resistance is the resistance between each input and ground.
What are 3 main characteristics of the practical op-amp model? How is it different from the ideal op-amp model?
Unlike the ideal op-amp model in the practical model op-amps take into consideration voltage and current limitations. Characteristics of a practical op-amp are: high voltage gain, high input resistance, and low output resistance.
What do op-amps do?
Used to perform primarily mathematical functions like: addition, subtraction, ect
Comparators usually use the ground (0 V) as a reference for their comparisons. How do you calculate the reference point for a comparator if a reference source other than ground is used like a battery or voltage divider. Use SDR2.56 to come up with a formula to calculate this.
V(REF) = (R(2) / (R(1) + R(2))) * V
How does a differential amplifier work?
When both inputs are grounded voltage and current for each output is the same (SDR2.44, picture on left). When bias voltage is applied to input 1 with input 2 grounded voltage at output 1 decreases and the current increases, while voltage at output 2 increase, while current decreases (SDR2.44 right most pic). Vice-versa happens when bias voltage is applied to input 2 (SDR2.45).
