Op amp Characteristics
Bandwidth of amplifier is a) Difference between upper cut-off frequency and lower cut-off frequency b) Sum of upper cut-off frequency and lower. It is required to increase the bandwidth to 25 kHz. feedback circuit will have (i) a series connection at the input, and (ii) a shunt (parallel) Quiz: Given a VCCS amplifier, what will be the topology of the feedback connections. To forward-bias a diode, the positive side of an external bias voltage is applied to the p region . CIRCUIT-ACTION QUIZ. 1. (a) . Voltage gain is the ratio of output voltage to input voltage. 3. .. RS open, no ground connection .. bandwidth.
- Amplifier Gain & Decibels
- Gain–bandwidth product
The logarithmic unit used is the decibel, which is one tenth of a Bel, a unit originally designed for measuring losses of telephone cables, but as the Bel is generally too large for most electronic uses, the decibel dB is the unit of choice.
Apart from providing a more convenient scale the decibel has another advantage in displaying audio information, the human ear also responds to the loudness of sounds in a manner similar to a logarithmic scale, so using a decibel scale gives a more meaningful representation of audio levels. The powers at various frequencies throughout the range are compared to a particular reference frequency, the mid band frequency.
Op-Amp Bandwidth, Gain Bandwidth Product & Frequency Response
Notice that, on the logarithmic frequency scale in Fig 1. Converting a power gain ratio to dBs is calculated by multiplying the log of the ratio by Where P1 is the power at mid band and P2 is the power being measured.
Voltage Gain in dBs Although it is common to describe the voltage gain of an amplifier as so many decibels, this is not really an accurate use for the unit. It is OK to use decibels to compare the output of an amplifier at different frequencies, since all the measurements of output power or voltage are taken across the same impedance the amplifier loadbut when describing the voltage gain between input and output of an amplifier, the input and output voltages are being developed across quite different impedances.
However it is quite widely accepted to also describe voltage gain in decibels.
To reverse the process, and convert dBs to a voltage ratios for example, use: Use the same formula for dBs to Current gain ratio, and to convert dBs to a power ratio, simply replace the 20 in the formula with An advantage of using dBs to indicate the gain of amplifiers is that in multi stage amplifiers, the total gain of a series of amplifiers expressed in simple ratios, would be the product of the individual gains: Large Signal Voltage Gain The large signal voltage gain is usually quoted in preference to the open loop voltage gain.
Figures for large signal voltage gain can cover a wide range for a given op amp, depending on design variant and factors such as minimum or maximum operating temperature. Closed Loop Voltage Gain In practise the huge gain of an op amp is greatly reduced by applying an appropriate amount of negative feedback.
In this way an impressively level response can be achieved, extending from DC 0Hz to any frequency up to about 1MHz or more, as well as the added benefits of reduced noise and distortion. The blue dotted line shows the response of the op amp with negative feedback.
Amplifier Gain and Decibels
The gain has been reduced to 20dB, a closed loop voltage gain Acl of x10, which has produced a flat response from 0Hz to about kHz. The graph of the open loop frequency response in Fig. For example, kHz bandwidth multiplied by a voltage gain of 10 also gives a Gain Bandwidth Product of: Then the Power Bandwidth becomes more relevant.
Maximum Differential Input This is the maximum voltage that can be applied between the two inputs, on some devices this can be equal to the supply voltage, but on others it can be considerably less. Input Resistance This is the resistance looking into the input terminals with the amplifier operating without feedback open loop.
Input Offset Current The currents flowing into the two inputs should ideally both be zero, but for practical op amps, although the input currents are still extremely small, they do exist and may also be different. Unequal currents cause different voltages at the inputs, and when this small difference in voltage is amplified, it causes the output to be other than zero.60. Op Amp Basics Part 4; Impedance, Bandwidth, and Gain
To overcome this effect an Input Offset Voltage can be applied between the inputs to correct the output voltage to zero. Temperature Coefficients Both the input offset current and input offset voltage are affected by changes in temperature, and tend to drift higher as temperature increases.