amplifiers. This design approach
was developed by Dr. Hung C. Lin
who worked for RCA in the early
days of transistors in the area of
developing applications. (Google
"Dr. Lin RCA" to find interesting
articles about him). This has come
to be known as the Lin
There are many variations on
this basic design. We have started
with (almost) the simplest of
them. Because this project
incorporates the first two stages of
most modern solid-state power
amplifiers, the same design can
be used to build a low to medium power audio amplifier.
Transistors are not inherently linear devices. Much can
be done to make them linear and minimize distortion. A
good start for an input stage is to use a pair of transistors
in differential mode (Q4 and Q5). A differential input
stage reduces the second harmonic to a very low level if it
is balanced accurately. The differential stage has two
inputs. This provides a very convenient way to connect
negative feedback to establish the gain of the amplifier
and to reduce distortion.
The pair of transistors in this stage work in opposition
to each other. If the signal current goes up in one, it goes
down by the same amount in the other. The non-linearities
in the two cancel each other. The extent of the
cancellation depends on the balance of the two. The
120 ohm emitter resistors reduce the gain of the stage
somewhat and also reduce the distortion. They make the
matching of the two transistors less critical.
Transistors Q1-Q3 at the top of the diagram comprise
a voltage reference and two current sources. Q2 is the
reference. It controls the emitter voltage on Q1 and Q3.
The current is adjusted by the value of the emitter resistor
in the current source transistor. The input differential
amplifier's current source is about 3 mA, set by the
220 ohm emitter resistor on Q1.
Using a current source provides a reduction in
distortion when compared to using a resistor. It also
greatly improves the "power supply rejection" — the
sensitivity to power supply ripple. For pennies, the
improvement in the circuit it provides makes it very much
The voltage reference is such that about 0.64 volts
appear across the 220 ohm resistor — a current of about
3 mA. This source provides what is commonly called the
"tail current" and the input stage is called a "long tailed
pair." An ideal current source would provide absolutely
constant current regardless of the load resistance over a
wide range. This simple circuit comes close.
Balance is critical so the load for the differential stage
is a "current mirror" (Q6 and Q7). It provides an active
load for the transistor and it balances the current between
the two transistors at zero signal, so the current is very
nearly the same in each. The current mirror has emitter
resistors of 68 ohms which compensate for small
differences in Vbe and beta. The addition of these two
inexpensive transistors and a pair of resistors can improve
the distortion level by a factor of 100.
Incidentally, I've built a number of amplifiers using this
input stage configuration and have never matched the
transistor pairs. The circuit is more or less immune to
variations in transistor beta and Vbe.
This first stage drives the base of Q8 which has its
emitter connected to the base of Q9. This arrangement
greatly enhances the current gain of this second stage.
The first stage is a "transconductance" stage which simply
means that it converts the input voltage to a current. The
second stage converts that current to a signal voltage. We
want a large voltage gain in this stage, so we use a current
source as the load.
This source is about 8 mA provided by Q3. It uses the
same reference voltage as the first stage's current source.
The 82 ohm emitter resistor sets the current. The AC
impedance of the current source is greater than half a
megohm, which makes the voltage gain high. This second
stage is called the voltage amplifier stage, or VAS for short.
In this simple amplifier, the output of the VAS drives
October 2013 45
• Transistors Q1-5 2N5551 NPN
• Transistors Q6-9 2N5401 PNP
• Output transistors Philips
• Resistors 1/4 watt carbon film
• Aluminum electrolytic
capacitors voltages as marked
• 100 pF capacitor must be NPO
ceramic or mica