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FAQs: Amplifiers (Part 1)

Posted: 09 Dec 2008 ?? ?Print Version ?Bookmark and Share

Keywords:FAQ? amplifier? power supply?

My op-amp is oscillating. What should I do?
The frequency of the oscillation is a very important clue. Frequencies near or above the op-amp's GBW generally show instability in the output stage, usually caused by capacitive loading on the output or poor supply bypassing. Also try adding a 0.1?F ceramic bypass capacitor to the power supplies.

If the frequency is within the operating range of the circuit (below GBW and near GBWP), and the overall circuit has a relatively high gain, it could be input-to-output coupling causing the feedback. Try shielding the input sections from the output sections and move input and output components away from each other.

If the frequency is below 10Hz (commonly referred to as "motorboating"), and the output is driving a relatively heavy load, it is usually an inadequate supply current, poor power supply bypassing, or the lack of a "star-ground" layout. This happens frequently with audio power amplifiers. Add larger supply bypass capacitors and make sure the load''s ground is returned directly to the power supply's ground.

Major causes of oscillations are poor grounding techniques, poor input to output isolation, and/or poor power supply bypassing.

What are the advantages of the BiCMOS process?
A BiCMOS process offers the IC designer the ability to use bipolar transistors where speed or voltage-noise levels matter while allowing the use of CMOS transistors where high impedance levels or substantial mixed-signal content is necessary.

What is Output High Voltage (VOH) for a comparator?
Output High Voltage (VOH) is the high DC output voltage of the comparator, with output driven high with specified output current. This specification is typically associated with comparators having totem-pole or push-pull outputs.

What is Output Low Voltage (VOL)?
Output Low Voltage (VOL) is the low DC output voltage with the output driven low with specified sinking current. This specification is typically associated with comparators having totem-pole or push-pull outputs.

What is Strobe "OFF" Voltage of a comparator?
Strobe "OFF" Voltage is the minimum voltage on the strobe terminal that will guarantee that it does not interfere with the operation of the comparator.

What is a Programmable Gain Buffer?
A Programmable Gain Buffer is an Op Amp with gain setting resistors integrated on the die allowing possible gains of +1, +2, or -1 using simple external connections. These devices are ideal for minimizing external component count, minimizing signal lead lengths, and simplifying designs.

When considering the ideal AC performance of an op amp, the Bode plot (gain vs. frequency response) is a one pole system. What is the rate at which the gain rolls off, in dB/decade?
In a one-pole system, the gain rolls off (or declines) at 20dB per decade. This is also 6 dB per octave. This is true for any one-pole response (ie: a simple RC filter or an ideal op amp). However, because the op amp has additional high-frequency poles, the phase shift will start to increase as the frequency approaches the op-amp unity gain frequency.

I am using a CMOS op-amp as an output driver. Although the circuit works well, I have noticed that if I use a long (1 Meter) shielded cable, the op-amp oscillates at around 1MHz with no input signal. If I shorten the cable to 10 cm, the oscillation subsides. What is causing this?
Some op-amps have trouble driving direct capacitive loads, such as a hunk of shielded cable which is a good capacitive load. 1 Meter of coax cable is about 60-100pF. Try placing a 50 to 500 ? resistor between the op-amp output and the cable. The datasheets for CMOS op-amps have a section on how to compensate for capacitive loads.

What is the difference between an op amp's "common mode voltage" and "input voltage range"?
While similar, these terms do not mean exactly the same thing. "Input voltage range" is the range of acceptable voltages applied to EITHER of the input pins. "Common Mode voltage" is a voltage applied to BOTH inputs simultaneously. Remember that an op-amp is supposed to REJECT common mode voltages and only amplify the difference BETWEEN the input pins.

What's the difference between a voltage feedback amplifier and a current feedback amplifier?
The internal circuitry of the two types of op-amps are different, and these two types of op-amps are not necessarily interchangeable in a given configuration. Voltage feedback op-amps are constrained by their internal design to have very low input bias currents, but the differential input voltage is unconstrained internally and is only constrained when external feedback applied around it. Conversely, for current feedback op-amps, the differential input voltage is constrained internally, but the input bias current is unconstrained by the internal design to be low, and is only constrained when external feedback applied around it. Although most colleges do not yet teach the basics in current feedback amplifiers, there are many advantages to using them, especially in high speed applications.

What's the difference between "Open Loop Gain" and "Closed Loop Gain"?
"Open Loop Gain" is the actual gain "inside" of the op-amp without feedback, usually anywhere from 1,000 to 10 million. "Closed Loop Gain" is the gain of the overall circuit, with feedback, and is selected by the user by choosing the appropriate feedback resistor ratios. This is your "gain of +10" or "gain of -2"

What does "Avol" stand for, and what is it?
Avol stands for "open loop voltage gain." The "A" is the symbol for gain. The "v" is a subscript that indicates voltage gain, as opposed to current gain. The "ol" is also a subscript and stands for open-loop. The open-loop voltage gain is the voltage gain of the amplifier (Vout/Vin) without feedback, compensating for any errors due to offset voltage

What are Differential Phase and Differential Gain?
"Differential Gain" and "Differential Phase" (DG/DP) are video measurements, and are a standard of measurements in the broadcast field. These measurements are the change in the amplitude, or phase, of the video signal over a stepped video waveform (think of it as delta gain or delta phase vs. Vout). The standard test signal used is a six-step monochrome video test pattern with a steady chroma subcarrier (NTSC 3.579MHz / PAL 4.2MHz) applied. The resultant video waveform resembles a six stair-step ramp (0 to 100% brightness) with the color subcarrier "fuzz" riding on top.

For differential phase, the test instrument establishes a phase lock with the colorburst reference signal, then compares the phase of the subcarrier "riding" along on each "step" of the stairstep, and displays the phase error for each "step". A good video amplifier will cause less than 0.1 degrees error. For differential gain, the test instrument compares the stairstep to a known amplitude reference, and displays the results. A good amplifier will have less than 0.1% of gain shift.

What does "CMR" stand for, and what is it?
CMR stands for Common Mode Range. The Common Mode Range is also known as input voltage range, and is a measure of the range of input voltages that the op-amp can accept at its input pins. This is usually specified with respect to the supply rails.

What is -3dB Bandwidth (or small signal bandwidth, SSBW)?
-3dB Bandwidth (or small signal bandwidth (SSBW)) is the frequency at which the closed loop amplifier small signal magnitude response is 3dB below its nominal value at low frequency. It is sometimes specified for various signal amplitudes.

What is Closed Loop Gain?
Closed Loop Gain is the ratio of an output voltage change to an input voltage change after feedback and input networks are added. Usually, external resistors are used to set this parameter.

What is Common Mode Input Resistance?
Common-Mode Input Resistance is the ratio of the common-mode input voltage change to the inverting or non-inverting input current change.

What is Common-Mode Rejection Ratio (CMRR)?
Common-Mode Rejection Ratio (CMRR): The ratio of differential voltage amplification to common-mode voltage amplification. It is measured by determining the ratio of a change in input common-mode voltage to the resulting change in input offset voltage change. CMRR(dB) = 20log 10 (DVCM /DVOS)

What is Common-Mode Voltage Range (Vcm)?
Typically the range of voltages on the input terminals for which the amplifiers' performance is.

What is Current Feedback?
Current Feedback is a technique used in Current Feedback Amplifiers, which generates an output signal in response to the current flowing into the inverting input node (transimpedance gain function). This topology offers operational advantages in certain areas, compared to the traditional voltage feedback.

What is Differential Gain and Phase?
Differential Gain and Phase: Differential Gain refers to change in gain with output input and Differential Phase refers to change in phase with input level. Both parameters are used in video broadcast applications as a measure of consistency of video.

What is Differential Input Resistance?Differential Input Resistance: The ratio of the differential input voltage change to the input current change.

What is Gain Bandwidth (GWB)?
Gain Bandwidth (GWB): The open loop gain times the frequency at a specified frequency higher than the first pole.

What is Gain Bandwidth Product?
Gain Bandwidth Product is an Arithmetic Product of a given input frequency and the op-amp open loop gain at that frequency (usually specified in MHz, voltage feedback amplifiers only.) For an ideal op amp, this is a constant for all frequency after the dominant pole frequency, but other poles and zeroes in the forward path could make the number vary with frequency.

What is Gain Flatness?
Gain Flatness is specified as "Peaking" and "Rolloff" numbers in dB over a given frequency band. It is a measure of an op amp's closed loop frequency response gain flatness. Phase margin, Gain margin, and sufficient loop gain are the most important parameters affecting these specifications.

What is Gain Margin (Cm)?
Gain Margin (Cm ): Open loop gain at the frequency where the phase between inverting input and output crosses zero.

What is Harmonic Distortion?
Harmonic Distortion is unwanted spurious signals generated at the output of an amplifier due to non-linearity in the signal flow path. With sinusoidal input, these spurs will occur at integer multiples of the input frequency (e.g. 2nd harmonic, 3rd harmonic).

What is Input Current (IB or Iin)?
The Input Current specification is the average of the currents drawn by the two input pins. Input current is also often called "bias current".

What is Input Current Noise (in)?
Input Current Noise (in ): The equivalent current noise applied in parallel with the input of the noiseless amplifier.

What is Input Impedance (Zin)?
Input Impedance (Zin ): The ratio of input AC voltage to input AC current.

What is Input Offset Current (Ios)?
Input Offset Current (Ios): The difference of the currents between the two input terminals.

Source: National Semiconductor

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