Comparing Op-Amp Parameters: Ideal vs A & B

[topcat123]

Homework Statement

1.
a) Contrast the relative performance of each op-amp in terms of these parameters and state how they compare to that of the ideal op-amp.
b) TABLE B gives two descriptions of operational amplifiers. State, with justifications, which description most closely matches the op-amps of TABLE A.
c) State, with reasons, which op-amp in TABLE A would be most suitable for the applications listed in TABLE C.
d) TABLE D shows the internal circuitry of two op-amps. Identify, with justification, which op-amp circuit is most likely to belong to which op-amp in TABLE A.

Table B
Description 1
A low cost, high speed operational amplifiers with an internally trimmed input offset voltage (BI-FET IITM technology). They require low supply current yet maintain a large gain bandwidth product and fast slew rate. In addition, well matched high voltage input devices provide very low input bias and offset currents.

Description 2
A dual amplifier for the amplification of low level signals in applications requiring optimum noise performance. Each of the two amplifiers is completely independent, with individual internal power supply decoupler-regulator, providing 120 dB supply rejection and 60 dB channel separation. Other outstanding features include high gain (112 dB), large output voltage swing and wide power bandwidth. The amplifier operates from a single supply across a wide voltage range. The amplifier is internally compensated with the provision for additional external compensation for narrow band applications

Table C
Application 1 Fast D/A converters.
Application 2 Audio pre-amplifier.

Homework Equations

None

The Attempt at a Solution

a)
The ideal op-amp will have infinite input resistance, zero output resistance and infinite gain
The two op-amps compare to this below.
Op-amp A has an input resistance of 200kΩ, an output resistance of 150Ω and a gain of
160,000 V/V
Op-amp B has an input resistance of 1TΩ, an output resistance of 10Ω and a gain of
300,000 V/V

b)?

c)
I think A because of the fast slew rate and because of the 15mhz band width.

d)?

Any help would be appreciated.

Thanks

 

[Merlin3189]

I've added some comments in red.

.
a) Contrast the relative performance of each op-amp in terms of these parameters and state how they compare to that of the ideal op-amp.

The ideal op-amp will have infinite input resistance, zero output resistance and infinite gain
The two op-amps compare to this below.
Op-amp A has an input resistance of 200kΩ, an output resistance of 150Ω and a gain of
160,000 V/V
Op-amp B has an input resistance of 1TΩ, an output resistance of 10Ω and a gain of
300,000 V/V
So, how would you say they compare? Is A or B closer to the ideal?

b) State, with justifications, which description most closely matches the op-amps of TABLE A. ?
Description 1
A low cost, high speed operational amplifiers with an internally trimmed input offset voltage (BI-FET IITM technology). They require low supply current yet maintain a large gain bandwidth product and fast slew rate. In addition, well matched high voltage input devices provide very low input bias and offset currents.
Description 2
A dual amplifier for the amplification of low level signals in applications requiring optimum noise performance. Each of the two amplifiers is completely independent, with individual internal power supply decoupler-regulator, providing 120 dB supply rejection and 60 dB channel separation. Other outstanding features include high gain (112 dB), large output voltage swing and wide power bandwidth. The amplifier operates from a single supply across a wide voltage range. The amplifier is internally compensated with the provision for additional external compensation for narrow band applications
This mentions other properties than those of an ideal op amp. So look at those properties in the table A

c) State, with reasons, which op-amp in TABLE A would be most suitable for these applications
Table C
Application 1 Fast D/A converters.
Application 2 Audio pre-amplifier.
I think A because of the fast slew rate and because of the 15mhz band width.
Sorry? Which do you think is A? Does application 1 use A or application 2?
Does the other application use the same op-amp, or the other op-amp?
It might be good to link your comments to what you think is needed for each application.
The descriptions (in part b) may also indicate the manufacturers thoughts about their application.

d) TABLE D shows the internal circuitry of two op-amps. Identify, with justification, which op-amp circuit is most likely to belong to which op-amp in TABLE A.
?
Well, what can you say about the two circuits? No numbers, but I notice two obvious features that are mentioned in the descriptions in Table A.

 

[topcat123]

Thanks for your help Merlin3189. This is where I am at so far.

(a)

The ideal op-amp will have infinite input resistance, zero output resistance and infinite gain

The two op-amps compare to this below.

Op-amp A has an input resistance of 200kΩ, an output resistance of 150Ω and a gain of

160,000 V/V

Op-amp B has an input resistance of 1TΩ, an output resistance of 10Ω and a gain of

300,000 V/V

Op-amp B is clearly closer to the values of the ideal op-amp than that of op-amp A(b)
Description one has a internally trimmed input offset voltage (BI-FET IITM technology). A FET Input Stages will have low input bias currents and have a high input resistance.
Op-amp B has a high input resistance and the low input bias current, suggesting this is description one.

Description two has a 120 dB Supply Rejection Ratio.
Op –amp A has the same supply rejection ratio 120 dB

(c)
my thinking is a Fast D/A converter dose not need a wide bandwidth but the audio pre amp will?

d)
Circuit 1 has a FET input stage so as in part b it will have a high input resistance making it op-amp B
I am still unsure on this part.

Circuit 2 has a BJT inputhavein a low input resistance making it fit with the specifications of op-amp A

thanks
ross

 

[Merlin3189]

I agree with a,b & d.
Part c) I don't know much about about myself.
Staring with the audio pre-amp: my first thoughts are that low noise is paramount, as that is what a pre amp is for - to raise a very low level signal to a useable level without adding too much noise. (Favours op=amp A)
Supply rejection might be useful to avoid adding hum. (Favours op=amp A)
Slew rate of both is more than adequate for audio frequencies and low levels. I estimate that a 20kHz signal at 5V needs about 1.2 V/μsec.
Similarly with the gain bandwidth: both can give a gain of 200 (46dB) over a 20kHz BW. 40+ dB seems common for audio preamps.
Input impedance might be an issue for high impedance crystal microphone, otherwise 200k for A is adequate.

As for D/A converter, I know even less! If the opamp is just a buffer after the DAC, I'd have thought the requirements were similar to the audio preamp, but wider bandwidth, so A might win out here.
But if the opamp were used in the DAC itself as a high speed switch, maybe slew rate and raw gain might be most important, favouring B.

But you'd better research this part yourself, unless someone more knowledgeable comes along, as I'm speculating from a very shaky base here!