Circuit Description; Block Diagram; Circuit Operation; Vertical Amplifier - Tektronix 221 Instruction Manual

CIRCUIT DESCRIPTION

The following circuit description begins with a discus­
sion of the instrument usingtheblockdiagram. Then each
circuit is described in detail, using detailed diagrams
where necessary to show the relationship of each stage
and the controls to that stage. The block diagram is shown
in Section 9.

BLOCK DIAGRAM

Signals to be displayed on the crt are applied to the
probe tip. The signal is then amplified by the vertical
amplifier. The vertical amplifier contains the vertical
position, attenuation, gain compensation, and input
coupling circuitry.
The Trigger Generator initiates the sweep signal
produced by the sweep generator. The input signal to the
trigger generator is selected from the vertical signal or
from an external signal applied to the EXT TRIG connec­
tor.
The Sweep Generator circuit produces a linear saw­
tooth signal that is amplified by the Horizontal Amplifier
circuit. The slope of the sawtooth generator is controlled
by the SEC/DIV switch. This circuit also produces the
unblanking signal coincident with the sawtooth to permit
display presentation.
The power supply circuit provides all the voltage
necessary for the operation of the instrument. Input power
is provided by internal batteries or a line source. This
circuit also provides a recharging circuit for the internal
batteries.

CIRCUIT OPERATION

The following is a description of the electrical operation
and relationship of the circuits in the 221 with a descrip­
tion of the circuitry most commonly used by Tektronix. If
more information is desired on the commonly used circuit,
refer to the following textbooks:
Phillip Cutler, "Semiconductor Circuit Analysis",
McGraw-Hill, New York, 1964.
REV. B, AUG 1974

Vertical Amplifier

The Vertical Amplifier circuit is divided into three
sections; the attenuators, the Preamplifier, and the Output
Amplifier. A block diagram of the Vertical Amplifier is
shown in Fig. 4-1. Schematics of the Vertical Amplifier are
shown on diagrams 1 and 3.
The input signals are applied to a 800 kilohm im­
pedance in the probe and a 200 kilohm attenuator
network, which provides the 1 megohm impedance for the
oscilloscope. The output of the attenuator is applied to the
input of the Preamplifier, which consists of three sections;
the buffer amplifier (Q132A and B, Q136A and B), a
resistive attenuator (R143-R156), and an operational
amplifier (Q162A and B, Q172A and B, and Q164). The
Preamp stage also provides Probe compensation,
Attenuator compensations, Step Attenuator Balance,
Volts/Div Balance, and Vertical Step Response (C167).
Output signals from the resistive attenuator are applied
to the positive input of the operational amplifier (Q162A
base). The output signal (from collector Q164) is then
applied to both the input of the output amplifier (base
Q422A) and, through feedback network R166and R167,to
the negative input of the operational amplifier (base of
Q162B). Transistor Q172A and B serve as dc biasing
circuitry for these amplifiers.
The Output Amplifier provides the final amplification
for the crt deflection plates and consists of three amplifier
stages. Transistors Q422A and B, Q426A and B, and Q424
are connected as an operational amplifier, with resistors
R416, R417, R418, and R432 serving as the feedback
network. It is otherwise similiar in operation to the
operational amplifier described for the Preamplifier stage.
The operational amplifier drives a differential amplifier
(Q442A and B), which in turn drives a dual output
amplifier. The Output Amplifier stage also provides the
circuitry for the bandwidth adjustment (C432), the VER­
TICAL POSITION control (R413), the Vertical Gain con­
trol (R417), and the VOLTS/DIV VARIABLE control
(R418). Transistor Q432 is connected as an emitter
follower and provides the internal trigger signal for the
trigger circuitry.
Section 4—221 Service
4-1