Digital to analog converter

Digital to analog converter
Digital binary signals are converted into analogue signals using digital to analogue converters.

Types of digital to analogue converter
Generally, two types of digital to analogue converters are used, the binary weighted
Input D/A converter and R/2R ladder D/A converter.

Binary weighted input D /A converter
The binary weighted input D/A converter is based on a summing circuit which sums the input current based on binary output and represents it as a voltage output. In the binary input method, a resistor network is used with resistor values representing the binary weights of the input bits of digital code. The binary is applied at the resistor inputs. A current will flow through the resistor of input voltage applied at the resistor inputs. A current will flow via the resistors in the input voltage logic applied is high. No current will flow via resistor if the input voltage applied is logic low. The magnitude of the current flowing via each resistor depends on the value of the resistor. The total current flow via each resistor adds up and flows via the feed back resistor Rf, which is connected via output and the inverted input of op – amp. The output voltage of op – amp is determine by the voltage drop across Rf resistance.

Figure: 4 bits digital to analogue converter

My Lab Experiment
I have implemented a D/A converter using 741C operation amplifier. The D/A is connected to a binary counter(7493). The binary counter input is drive using 1 Hz clock which is also created by my own.  

Figure: 4 bits digital to analogue converter


An electronic circuit which amplifies an input signal into much larger signal in magnitude at the output.

Types of amplifier
There are different ways to describe amplifiers e.g. by their inter stage coupling or by their frequency ranges.
Types of coupling
1. Capacitive coupling
The capacitor coupling transmits the amplified ac voltage to the next stage.
2. Transformer coupling       
The ac voltage is coupled through a transformer coupling to the next stage.
3. Direct coupling
There is a direct connection between the collector of the 1st transistor and the base of the 2nd transistor because the dc and the ac voltage are coupled and direct coupled amplifiers are sometimes dc amplifiers.

Ranges of frequency
Another way to describe amplifiers is by stating their frequency range e.g.  An audio refers to an amplifier that operates in the range of 20Hz to 20 KHz. On the other hand, a radio frequency amplifier is one that amplifies frequencies above the 20 KHz usually much larger e.g. The RF amplifiers in AM radio amplify frequency between 535 and 1605 KHz and the RF amplifiers in FM radios amplify frequency between 88 to 108 MHz. Amplifiers are also classified as narrow band / wide band. A narrow band amplifier works over a small frequency range like 450 to 460 KHz. A wide band amplifier operates over a large frequency range like 0 to 1 MHz. These amplifiers are usually tuned RF amplifiers which mean that their ac load is a high Q resonant tank tuned to a radio station. A wide band amplifiers are usually untuned which means that their ac load is resistive.

Characteristics of amplifiers

  1. Large amplification gain.
  2. Large input resistance.
  3. Small input resistance.
  4. Large bandwidth.

RC common emitter amplifier
The following figure shows the common emitter amplifier having two coupling capacitors and a bypass capacitor with voltage divider biasing;

Figure: Common emitter amplifier

re model of common emitter amplifier
The following figure shows the common emitter amplifier;

Figure: re model of common amplifiers

                                                                         Zi = (Beta) re
                                                                         Av = – RL / re
                                                                          Ai = Beta

H-parameter equivalent circuit
The following is the h parameter equivalent circuit of common emitter circuit by replacing the transistor with the ac equivalent circuit with its parameter equivalent circuit.

Figure: h-parameter equivalent circuit

                                                                           Zi  = R1 || R2 || hie

                                                                           Zo = RC || 1 / hce                                 

                                                                           Av = – hre * Rc / hie

                                                                           Ai = hre * Rb / (Rb + hie)

                                                                           Ap = Av * Ai

Lab Experiment
The following figure shows the common emitter amplifier on a bread board.

Figure: Common emitter amplifier on bread board


Zener diodes

Zener Diode
In case of zener diode the differential voltage and current in the zener region of the characteristic curve is given by
                                                                      ΔV = rz. ΔI

Where     rz = inverse of the slope or incremental resistance of zener diode at operating point.

Figure:  Zener diode I-V Characteristic

The almost linear i-v characteristic of zener diode suggests that the device can be modeled as shown;

Figure: Zener diode model

Practice, values of knee voltage and Vz0 are considered same. From the model

                                                  Vz =Vzo +rz + Iz
                                                  When Iz > Izk
                                                  & Vz > Vzo 

Zener diode as shunt regulator
A regulator must posse’s two properties
1). No change in V0 .with any change in Vs .
2). No change in Vo with any change in IL. Therefore two parameters are related as
(1)               line regulation = ΔVo/ ΔVs
(2)                Load regulation = ΔV0/ ΔIL

 Another method to describe these regulations;

Figure: Zener as shunt regulator

Positive voltage regulator

Figure: Positive voltage regulator

Split supply regulator circuit

Figure: Dual supply regulator circuit