EE222 Spring 2002 
(This note is a slight modification of a note written in Fall 2001). 

Lab Experiment #7.  

Feedback from the TA.
*********************************************************************************  

Lab notebook.  

Part 2 (c)  

In the negative feedback amplifier circuit, we input a sine wave ( vi(t) ). 
We increase the amplitude of the input (Vimax) so that the output ( vo(t) ) 
saturates. Then we notice that a signal ( vB(t) ) appears at the negative 
input of the op-amp. If we record the amplitudes of the three signals, 
we observe the following relationship between the three values:   	

VBmax = Vimax - ( Vomax/G ),  	

where G is the gain of the feedback amplifier. 

***Addition on April 01, 2002***: I wrote the above relationship based on 
experimental results. A more accurate relationship based on theoretical 
argument can been found in the note written by Mr.Steve McFadden.    


Part 3(b)  

We are asked to build a two-input summing circuit and apply an input (sinusoidal 
or triangular) voltage, and adjust Rf to obtain a very high gain. What is happening 
to the output and why? In fact, how high a gain are we expected to have to notice 
any effect? I will answer this question here. As you would have completed Lab 
Experiment #8 when this feedback reaches you, you will be able to appreciate 
the answer.  

When v1(t) = v2(t), the 2-input circuit is effectively a single-input circuit 
with a gain G = - Rf/(R1 || R2). We are given that R1 = 10 kiloohm, and 
R2 = 20 kiloohm. So, G = - Rf*(3/20). From Lab #8, we know that the 
gain-bandwidth product for this op-amp is about 200 kHz. The input signal 
v1(t) = v2(t) has a frequency of 200 Hz if it is a sine wave (and 100 Hz if 
it is a triangular wave). So, when the gain is about 1000, the bandwidth of 
the feedback amplifier is about 200 Hz. If we increase the |G| to say 2000, the 
cut-off frequency (same as bandwidth) is about 100 Hz and our v1(t) = v2(t) will 
be amplified less than 2000 times, and maybe only 1500 or 1000 times. Note that 
to observe this amplification, we need to decrease the magnitude of the input.   

In short, at very high gains (of the order of about 2000), the amplification of 
the input signal by the 2-input summing circuit is much less than what we would 
expect with the knowledge we have at this stage of EE222.    



Reports.  

1) The HA17741 is the first integral circuit (IC) that we have encountered on 
this course. Integral circuits have multiple pins ( > 2 ). It is a good practice 
to include a diagram showing the pin arrangement of the IC at least in the first 
report that the IC is used. For example, to which pins do the +Vcc and -Vcc 
voltages go? Which pins are involved in setting up the offset nulling circuit? 
Which pins have signals on them?