Lab 5 wps

							EE 210 Section 6

Laboratory 5 Experiment 2: Voltage Follower/Buffer - Loading Effect



Experiment Date: February 27th, 2007

Submission Date: March 6th, 2007




Jon Stephens       _________________________



Partner: Jessica Van Ness




Abstract:

       This experiment’s purpose is to study how a buffer is used. More importantly,

how it is used to overcome the loading effect. The measured dc offset is higher then it is

supposed to be, therefore we can say that the buffer ignores the loading effect resistance.

We can come to this conclusion by comparing our results to a similar experiment in

laboratory 2. In this experiment, the dc voltage and dc current are both higher than that

of laboratory 2.

Experimental Plan
Bill of Parts:


Part Name            Description                   Quantity
Agilent 33220A
Function GeneratorProvides ac and dc voltage           1
Tektronix         Reads the signal from the
Oscilloscope      function generator                   1
Resistors(Ohms)   68k, 4.7k, 60, 360, 51             1 each
                  Splits the output of the
T-Connector       function generator                   1
50 Ohm Terminator 50 ohm load on the t-connector       1




Schematic of Network:

Figure 1: Inserting a Buffer.




Parameters: RL = 68k ohms, 4.7k ohms, 680 ohms, 360 ohms, or 51 ohms.



Comments on Implementation: Set up Figure 1 and measure the ac amplitude and dc

offset using all 5 values for RL.



Numbered List of Instructions:

1) Construct Figure 1 on the breadboard. For Vs use the function generator set at 5

Vpp, 1 V dc offset, 100 kHz, and 50 ohm load. Connect outputs to the oscilloscope.
2) For each resistor value, measure the ac amplitude and dc offset.

3) Calculate the corresponding ac and dc current using Ohm’s Law.

4) Compare the measurements with that of laboratory 2.



Data and Calculations

Table 1: AC vs. DC

Nominal     Actual      vL ac vL dc       iL ac   iL dc
Load (ohms) Load (ohms) (Volts) (Volts)   (mA)    (mA)
    68k         71.2     4.96     1.3      0.07    0.02
    4.7k        4.77     4.92     1.3      1.03   0.272
    680         663      4.92     1.3      7.42    1.96
    360         355      4.88     1.3     13.77    3.66
    51          50.3     2.84    0.76     56.46   15.12




Table 2: LAB 2 AC vs. DC

Nominal(ohms) Actual (ohms) Vac(V) Vdc(V) Iac(mA)         Idc(mA)
     68k          67.8k      4.88   0.92     0.072          0.019
     4.7k         4.66k      4.8     0.9      1.03          0.193
     680          673        4.68   0.888     6.95           1.32
     360          358        4.56   0.853    12.74           2.38
     51           50.6       3.2    0.585    63.24          11.56




By comparison, dc is higher in this lab than it was in laboratory 2. Other numbers are

similar.



Calculations:

V = IR

I = V/R

I = 2.84 volts / 51 ohms
I = 56.46 mA

Graph 1: RL = 68k ohms




Graph 2: RL = 51 ohms
Observations

          As the resistor values go down, then the voltage drop is more. However, the

voltage drop is only significant when using the 51 ohm resistor. Therefore, the current is

significantly higher when using the 51 ohm load. The dc voltage readings are different

when compared to that of laboratory 2. The dc voltages are higher, therefore the current

is also higher. The graph of the 51 ohm resistor shows a flat top on the output voltage,

which means that this is the maximum and it cannot go above that.



Conclusions

          The buffers main job is to make Vin = Vout. This makes a gain of 1 in the

op-amp. The buffer overcomes the loading effect of the function generator. The

experiment in laboratory 2 does not overcome this loading effect, therefore there is a dc

voltage drop. This buffer can help if you do not want the dc voltage to drop, and keep it

higher.