1
Q
State the characteristics of an ideal Amplifier compared to practical amps.
A
- Infinite voltage gain, A(V) / very high voltage gain
- Infinite input impedance, Z(in) / very high input impedance
- Zero output impedance, Z(out) / very low output impedance
- Infinite bandwidth, BW / wide bandwidth
- Zero Distortion (or Input/output relationship = linear)
( i.e. V(o) is proportional to V(i) ) - However, Peak-to-peak output voltage usually limited to
slightly less than the two supply voltages. - Output current also limited by internal restrictions such as power dissipation and component ratings
2
Q
What are the different properties of an Amplifier?
A
- Gain: The gain of an amplifier is a multiplier that exists between the input and output that multiplies the input voltage
- Bandwith: The gain of the amplifier is not uniformed – it falls after a certain frequency. The range of frequencies over which a useful gain is available is called the bandwidth of the amplifier
- Impedance measured between input terminals and output terminals.
- Distortion: While output signal > input signal, it must also
retain the same shape as the input signal – linear input and output relationship
3
Q
What are the different operational modes of amps?
A
- Single-ended input mode has one input connected to the input signal and the other connected to ground.
- The inputs of a differential input circuit are connected to different signal sources
- In the case of common-mode input, both inputs are connected to the same signal source.
4
Q
What are output voltage limits?
A
- The upper limit is called the positive saturation voltage (+Vsat) and it is approximately within 1V of +V.
- The lower limit is called the negative saturation voltage (-Vsat) and it is approximately within 2V of -V.
- With a supply voltage of +/-15V, +V(sat) =+14V and -V(sat) = -13V. Therefore V(o) is restricted to a symmetrical peak-to-peak swing of +/-13V.
5
Q
What is negative feedback of op amps and why?
A
- Negative feedback is the process whereby a portion of the
output voltage of an amplifier is returned to the input with a phase angle that opposes (or subtracts from) the input signal. - Extremely small input voltage drives the op-amp
into saturation (its limits), Negative feedback loop prevents op-amp from saturating - Negative feedback creates an effective reduction in gain
by taking a portion of the output and is fed back to inverting input so that the op-amp can function as a linear amplifier.
6
Q
How does negative feedback affect the properties of op amps?
A
- Voltage gain is reduced to desired value
- Input impedance can be increased or decreased to desired value
- Output impedance can be reduced to desired value
- Bandwidth much wider
7
Q
What is the effects of negative feedback?
A
- The input voltages at the inverting and non-inverting terminal are forced to be equal.
- The input currents to both the inverting and non-inverting terminals are zero ( ie. no input current flowing into the op-amp)
8
Q
What are some uses of a voltage follower?
A
- Has voltage gain of 1 as V(in) = V(out)
- Capable of driving a relatively low resistive load due to its very small output impedance
- Often used as buffer circuit to match a high internal resistance signal source to alow resistive load
9
Q
How to tell between a non-inverting and inverting amplifier?
A
If the input is applied to the (−) terminal, the amplifier is inverting.
If the input is applied to the (+) terminal, the amplifier is non-inverting.
10
Q
What is a summing amplifier?
A
- Variation of the inverting amplifier
- Has two or more inputs connected in parallel to inverting terminal and its output voltage is proportional to the negative of the algebraic sum of its input voltages
- Can be used as averaging amplifier where value of input resistors is equal to nR(f) where n in number of inputs
11
Q
What is a zero-level and non-zero level comparator?
A
- Former compares input with ground and latter compares input with a reference voltage.
- V(out) = A (V(+) - V(-) )
- V(+) > V(-) => V(out) = +V(sat)
- V(-) > V(+) => V(out) = -V(sat)
12
Q
How to tell between positive and negative feedback?
A
- When feedblack loop is connected to (-) terminal, negative feedback.
- When feedback loop is connected to (+) terminal, positive feedback.
13
Q
How to reduce noise in comparators?
A
- To make the comparator less sensitive to noise, applying positive feedback in an op-amp circuit, hysteresis. -=> higher reference level when the input voltage goes from a lower to a higher value
- V(ref) is connected to positive feedback loop and V(in) to (-) terminal
- The two reference voltages are referred to as the upper trigger point (UTP) and the lower trigger point (LTP)
- V(UTP) > V(In) => V(out) = +V(sat)
- V(In) > V(LTP) => V(out) = -V(sat)
- When input exceeds V(UTP), it switches to maximum negative voltage.
- When input goes below V(LTP), its switches to maximum positive voltage.
