The Frequency Response of Opamp

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  • Topic: Operational amplifier, Operational amplifier applications, Amplifier
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To compute and measure the bandwidth of non-inverting op-amp circuits as a function of gain.


Resistors: one 200 Ω, one 10 kΩ, one 33 kΩ, one 68 kΩ

ISO –Tech Digital Oscilloscope IDS 8062
Pintek 3 Mhz Function Generator
Bread Board
Schmit Trigger
Fluke 116 True RMS Multimetre


There are two basic forms in which an Op Amp (Operational Amplifier) can be used as an amplifier: 1) Inverting configuration and 2) Non inverting configuration. No amplifier has infinite frequency response: There is a limit to the frequency any amplifier will handle. The Op Amp has high gain and very high input resistance and very low output resistance. The typical frequency response for a typical op-amp is shown as the curve marked open-loop gain in Figure 1. The plot shows the gain plotted as a function of frequency in kHz. The “frequency response” of any circuit is the magnitude of the gain in decibels (dB) as a function of the frequency of the input signal. The gain expressed in dB is 20 log10|G|. The frequency response of an op-amp is a low pass characteristic (passing low-frequency signals, attenuating high-frequency signals), Figure 1.

Gain (log s cale)

G -3 d B point

B Fre q (Hz )

Figure 1: Frequency response of op-amp.

The bandwidth is the frequency at which the power of the output signal is reduced to half that of the maximum output power. This occurs when the power gain G drops by 3 dB. In Figure 1, the

bandwidth is B Hz. For all op-amps, the Gain*Bandwidth product is a constant. Hence, if the gain of an op-amp is decreased, its operational bandwidth increases proportionally. This is an important trade-off consideration in op-amp circuit design. The diagram below of the op-amp configuration has a path from the output of the op-amp back to its inverting input. When the output is not “railed” to a supply voltage, negative feedback ensures that the op-amp operates in the linear region (as opposed to the saturation region, where the output voltage is “saturated” at one of the supply voltages). Amplification, addition/subtraction, and integration/differentiation are all linear operations. Note that both AC signals and DC offsets are included in these operations, unless we add a capacitor in series with the input signal(s) to block the DC component


Figure 2


Frequency Response of the inverting Amplifier:

1. The values of the resistors used in the practical were measured and recorded as shown listed in Table 1.

2. A inverting voltage-follower was constructed as shown in figure 2 with the power supplies set to 15 V. 3. The amplitude of Vin (input voltage) was adjusted to avoid saturation that is to avoid a distorted output Vout. The input voltage Vin was fixed at this constant and vout was measured for the frequency values of between 30Hz and 100kHz. The gain of the amplifier Vin/Vout was calculated for these frequencies and were recorded in table 2, 3 and 4 for Rf of 10kΩ, 33kΩ and 68kΩ respectively.

4. Rf was replaced with 33kΩ and 68kΩ the above procedure 3 was repeated. 5. The graphs log of gain (Av) against log of frequency was plotted for all Rf values mentioned in step four (4).


Figure 2

Table 1:

|Nominal value |220Ω |10kΩ |33kΩ...
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