Question 7.3: A certain op-amp has three internal amplifier stages with th...
A certain op-amp has three internal amplifier stages with the following gains and critical frequencies:
Stage 1: A_{v 1}^{\prime}=40 \mathrm{~dB}, f_{c 1}=2000 \mathrm{~Hz}
Stage 2: A_{v 2}^{\prime}=32 \mathrm{~dB}, f_{c 2}=40 ~\mathrm{kHz}
Stage 3: A_{v 3}^{\prime}=20 \mathrm{~dB}, f_{c 3}=150 ~\mathrm{kHz}
Determine the open-loop midrange dB gain and the total phase lag when f=f_{c 1}.
A_{o l(m i d)}^{\prime}=A_{v 1}^{\prime}+A_{v 2}^{\prime}+A_{v 3}^{\prime}=40 \mathrm{~dB}+32 \mathrm{~dB}+20 \mathrm{~dB}=92 \mathrm{~dB}
\phi_{t o t}=-\tan ^{-1}\left(\frac{f}{f_{c 1}}\right)-\tan ^{-1}\left(\frac{f}{f_{c 2}}\right)-\tan ^{-1}\left(\frac{f}{f_{c 3}}\right)
=-\tan ^{-1}(1)-\tan ^{-1}\left(\frac{2}{40}\right)-\tan ^{-1}\left(\frac{2}{150}\right)
=-45^{\circ}-2.86^{\circ}-0.76^{\circ}=-48.6^{\circ}
P R A C T I C E EXERCISE
The internal stages of a two-stage amplifier have the following characteristics: A_{v 1}^{\prime}=50 \mathrm{~dB}, A_{v 2}^{\prime}=25 \mathrm{~dB}, f_{c 1}=1500 \mathrm{~Hz}, \text { and } f_{c 2}=3000 \mathrm{~Hz}. Determine the open-loop midrange gain in dB and the total phase lag when f=f_{c 1}.