Philips AN1651 manual VIII. GAIN-BANDWIDTH VS Closed Loop FRE- Quency Response, Ix. Loop-Gain

Philips Semiconductors	Application note
Using the NE/SA5234 amplifier	AN1651

VIII. GAIN-BANDWIDTH VS CLOSED LOOP FRE- QUENCY RESPONSE

Figure 5 shows the small signal frequency response of the NE5234 versus closed-loop gain in dB. The test circuit is shown in Figure 6. The plot is taken from measured data and thus shows how each value of closed-loop gain coincides with the open-loop response curve. The NE/SA5234's open-loop gain response has a uniform 6dB/octave roll-off which continues beyond 2.5MHz. This factor guarantees each op amp in the IC a high stability in virtually any gain configuration. In making these measurements, dual supplies of

±2.5V were used in order to allow a grounded reference plane and no coupling capacitors which might cause frequency related errors.

A critical parameter which affects the reproduction quality of complex waveforms is the gain-bandwidth-product of the operational amplifier. Essentially, this is a measure of the maximum frequency handling characteristics of any operational amplifier for a given closed-loop gain. As is evident from the graph, the NE/SA5234 has a 2.5MHz unity gain cross-over frequency Ð much higher than most other low voltage op amps. For comparison, the μA741 has a gain-bandwidth-product of 1MHz, as do the LM324 and the MC3403.

aforementioned factors that affect the signal-to-noise ratio of the stage and optimizing the Loop-gain. For example, a voice-band audio stage which requires 3kHz bandwidth, should be limited to a closed-loop gain of 40dB for lowest distortion in the output signal. For higher quality audio applications requiring a 20kHz bandwidth, the closed-loop gain must be limited to 20dB. This results in a Loop-gain of 20dB at the highest signal frequency.

A second consideration in the list of frequency dependent parameters is the effect of amplifier slew rate. Not only is it frequency dependent but it is also a function of signal amplitude, as we shall see in the next section.

AOL

-6dB/Octave

LOOP

GAIN

ACL

fS fu SL00640

IX. LOOP-GAIN

The dynamic signal response of any closed-loop amplifier stage is a function of the Loop-gain of that particular stage. Loop-gain is equal to the open-loop gain in dB, at a given frequency, minus the closed-loop gain of the stage. The greater the Loop-gain, the lower the transfer function error of the device. Essentially, any parametric error is reduced by the factor of the Loop-gain. This includes output resistance and output signal voltage accuracy. It is good practice then to maximize Loop-gain to the degree that stage gain may be sacrificed for bandwidth. In some cases it is actually better to use two stages of gain in order to preserve signal quality than to use one high gain stage. Of course, there is a trade-off between the

Figure 12.

X. SLEW RATE RESPONSE

The slew rate of an operational amplifier determines how fast it can respond to a signal, and is measured in volts-per-microsecond. The NE5234 has a typical slew rate of 0.8V/μs. Let us see just what this means in terms of signal handling capability. If a sinusoidal input signal, VS, is used as reference, it is specified by its frequency and peak amplitude, VP as follows:

VS + VP sin (2pf t) (EQ. 11.)

2
	VPK = 1.096V
VOLTS PK	VPK = 630mV
	VPK = 100mV
0.02		2000000
2000	(Hz)
		SL00641

Figure 13. Slew Rate Limiting Amplitude vs Frequency

Slew Rate (SR) is the time-rate-of-change of the signal voltage during any complete cycle, that is over the range of 0 to 2π. This amounts to taking the time derivative of the sine wave which results in multiplying the cosine by the factor `2πf'.

An example of the trade off between signal amplitude and frequency is shown below for the NE5234 slew rate of 0.8V/μs. As shown in

Figure 13, the maximum allowable amplitude signal which can be reproduced is determined by the slew rate response line which gives peak output volts versus frequency in Hertz.

Mathematically, slew rate is determined, by the equation below, as the derivative of the sine wave signal. The resultant slew rate function changes with both frequency and amplitude.

1991 Oct 8