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% LaTeX technical info: see "A Document Preparation System" 2nd
%   edition, Leslie Lamport
% author of this document: Peter Stallinga

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\begin{center}
  \fbox{\LARGE Project 4}\\
  {\tiny P.\ Stallinga, 23.05.2001}\\
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\noindent
The circuit consists of the following elements:
\begin{description}
\item[I] \textbf{opamp}. The opamps supply the voltage
  amplification. The first opamp is a tension-follower which ensures
  the high input resistance. The second opamp supplies the actual
  amplification with $A_v = R_f/R_1$.\\
\item[IIa and IIb] \textbf{current sources}. The input of the power
  stage needs to be polarized at +1.4 V (top leg) and -1.4 (bottom
  leg). For this we use two current equal sources and two common-bases.
\item[IIIa and IIIb] \textbf{Common Bases}. The common bases have to
  make sure that the power stage is biased well. For the top half we
  need 2 x $V_{be}$ = + 1.4 V at the input. The output of the opamp
  is 0 V (DC) and hence we need a level shifter of 1.4 V. This is done
  by choosing $R_{B1}=R_{B2}$ in the CB. To the bottom half applies
  the same: with equal base resistors we get -1.4 V at the entrance of
  the power stage.
\item[IV] The last part is a current amplifier.
  This has no voltage amplification but can supply a large current
  needed for an 8 $\Omega$ load at the output. This stage is
  symmetrical with each side taking care of half of a wave. Each side
  is comprised of a Darlington pair, which has a huge current
  amplification, $(\beta+1)^2$, but no voltage amplification. The
  output resitance of this stage is basically the emittor resistance:
  $R_E + r_e + R_B/(\beta+1)^2$, with $R_B$ the resistance at the base of
  the first transistor (k$\Omega$'s).

\end{description}

\vspace{3cm}
\noindent
\emph{\small \textsc{Annex:}\\
Simulations with Electronic Workbench 5.12 and PSpice 8.0}
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