\section{E-8: Transistors}\label{e10_transistors}

\noindent OBJECTIVE:\begin{enumerate}
\item[] \latex{\vspace*{-.1in}} 
To experiment with a transistor and demonstrate its basic
operation.
\end{enumerate}

\noindent APPARATUS:
\begin{enumerate}
\item[] \latex{\vspace*{-.1in}}An {\bf npn} power transistor; 
dual trace oscilloscope 
\& manual;  signal generator \& frequency counter; 
power supply ($\pm $15 V fixed/$\pm $9 V variable); 
circuit plug board \& component kit; 
two digital multimeters (DMM); differential 
amplifiers.
\end{enumerate}

\noindent INTRODUCTION:\begin{enumerate}
\item[] \latex{\vspace*{-.1in}} 
Our junction transistor (Fig.~1) is like two back to back 
{\em np} diodes (see E-8 Part C,~\#3).
Hence there are two possibilities, an {\em npn} 
transistor and a {\em pnp}
transistor.  Our {\em npn} transistor has a central p-type layer (the 
{\em Base}) 
between two n-type layers 
(the {\em Emitter} and {\em Collector}).  There are other type transistors 
which we will not discuss,
(e.g.~a {\em MOSFET:} Metal-Oxide 
Semiconductor-Field-Effect-Transistor). \html{\\}
%\begin{figure}[h] \centering
\begin{center} 
\latex{\hspace*{.0in}\includegraphics[height=1.55in]{figs/e10-01.eps} \\ Figure 1}
\html{ {\bf Figure 1} \\
      \includegraphics[height=2.in]{figs/e10-01.eps}}
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\latex{\vspace*{-.1in}}
\noindent For an {\em npn} transistor the {\em collector is positive 
relative to the emitter}.  
The base-emitter circuit acts like a diode and is normally conducting 
(i.e.~forward-biased).  The base-collector circuit also acts as a diode but is 
normally non-conducting (reverse biased) if no current flows in the 
base-emitter circuit.  However when current 
flows in the base-emitter circuit, the high concentration gradient of 
carriers in the very thin base gives an appreciable diffusion current to the 
reversed biased collector.
The resulting 
collector current $I_{c}$ depends on the base current $I_{b}$.  We write 
I$_{c} = \beta I_{b}$ where $\beta $ is the current amplification factor.


\framebox[5.2in]{\parbox[b]{5in}{\smallskip {\em PRECAUTIONS:} 
Although our power transistor is fairly indestructible, its 
characteristics are temperature sensitive.  Hence avoid exceeding the 
voltages suggested; also leave the power supply off when not making 
measurements; read currents and voltages to two significant figures only. 
\smallskip \par }}
\end{enumerate}

\noindent SUGGESTED~EXPERIMENTS: 

\html{\begin{tabular}{p{2.5in}l} }
\latex{\vspace*{-0.15in}\hspace*{.3in}\parbox[b]{0.35\linewidth}}
{\hspace*{-.2in}1.{\em ~CURRENT~Amplification:}  Measure $\beta $ for various 
emitter to collector voltages, $V_{ec}$.
Set up the circuit as in Fig.~2.  
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\latex{\includegraphics[height=1.6in]{figs/e10-02.eps}\vspace*{-.1in}
\begin{center} \hspace*{.9in} Figure~2 \end{center}}
\html{
\begin{center} {\bf Figure~2} 
\includegraphics[height=1.8in]{figs/e10-02.eps}
\end{center}
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\html{ \cr \end{tabular}  }


\begin{enumerate}
\item[]
\noindent Start with the voltage divider 
completely counter-clockwise so that the emitter-base voltage $V_{eb}$ is a 
minimum.  With the variable power supply set to 3 V, record the $I_{b}$ 
measured on DMM$_1$ and the $I_{c}$ on DMM$_2$

\noindent Repeat the readings for ten reasonably spaced (higher) 
settings of the voltage divider (i.e.~higher $V_{eb}$ and hence higher $I_{b}$.

\noindent Calculate $\beta (=I_{c}/I_{b}$) and plot it against $I_{b}$.  Over what range of 
I$_{b}$ is $\beta $ reasonably constant?
Repeat the above but with emitter to collector voltage $V_{ec}$ now at 7 V.
\item[2.] (Optional) {\em  MEASUREMENT~OF~$I_{c}$~vs~$V_{eb}$:}
Remove the multimeter DMM$_1$ (used as a microammeter) from the 
circuit of Fig.~2 and use it instead as a voltmeter to measure
the emitter to base voltage $V_{eb}$.  With the variable power supply set to 
give 7 V for $V_{ec}$, use the voltage divider to vary $V_{eb}$. 
Read and record both $V_{eb}$ and $I_{c}$ for 
10 reasonably spaced values of $V_{eb}$.
Plot $I_{c}$ vs $V_{eb}$.  The results are very similar to a diode curve (as 
expected since $I_{c}$ is proportional to $I_{b}$ over the region where 
$\beta $ is a constant).
\item [3. ]{\em VOLTAGE~Amplification:}  By adding a large load resistor 
in series with the collector, one can convert the current 
amplification $\beta $ observed earlier into a voltage amplification.
Hook up the circuit plug board as in Fig.~3.\smallskip


%\begin{figure}[h] \centering 
\begin{center}
\latex{\hspace*{.30in}\includegraphics[height=2.65in]{figs/e10-03.eps} \\ Figure 3}
\html{{\bf Figure 3} \\ \includegraphics[height=3.in]{figs/e10-03.eps}}
\end{center}
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\noindent The input voltage to the transistor $V_{in}$ includes the drop across
the $4.7~\mbox{k}\Omega $ 
protective resistor.  The voltage gain is $\mbox{G}=\Delta V_{out}/\Delta V_{in}$.
Record input voltages $V_{in}$ and output voltages $V_{\mbox{\tiny out}}(=V_{ec}$) for ten 
reasonably spaced values of $V_{in}$ from 0 to 5 volts.

\noindent Graph V$_{out}$ vs V$_{in}$ and calculate the voltage gain G at the steeply changing 
part of your graph.  The value of V$_{in}$ at the center of this region we 
call the ``operating voltage'' of the amplifier.
How would the gain change if the load resistance was $2.2~\mbox{k}\Omega $ 
instead of 
$10~\mbox{k}\Omega $?
\item [4.] {\em Distortion~effects~when~amplifier~is~overdriven:}  Set 
up the circuit 
plug board as in Fig.~4. Connect Y$_1$ and Y$_2$ to the scope.  To 
connect the signal generator to the board, 
use the BNC to banana plug adapter and 
remember that the side with the bump goes to ground.
\end{enumerate}

\begin{enumerate}

\item[]
%\begin{figure}[h] \centering 
\begin{center}
\latex{\hspace*{.60in}\includegraphics[height=2.85in]{figs/e10-04.eps} \\ Figure 4}
\html{{\bf Figure 4} \\\includegraphics[height=3.2in]{figs/e10-04.eps}}
\end{center}
%\end{figure}

\noindent Adjust the voltage divider until the $V_{in}$ is close to the ``operating 
voltage'' found in \#3.

\noindent Vary (and record) the amplitude of the input signal 
from small values to those which overdrive the amplifier
and produce considerable distortion in the output Y$_2$.

\noindent Observe and record the effect of changing V$_{in}$ 
to values outside of the 
operating range (where $\beta $ is constant).
\end{enumerate}
