Differnce bw bjt & fet


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Differnce bw bjt & fet

  1. 1. Class No 09 Zunaib Ali AssignementNo :01 DIFFERENCE BETWEEN FET & BJT ON THE BASIS OF APPLICATIONS Bipolar Junction Transistor Field Effect Transistor 1) A bipolar transistor requires a small amount of current flowing to keep the transistor on. While the current for one transistor may be negligible, it adds up when millions are switching simultaneously. The heat dissipated on bipolar limits the total number of transistors that can be built on the chip 2) A BJT will consume more power in the on- state.it cannot switch with less than a 0.3V voltage drop. 3) BJT’s function as regulators of currents as small current is regulating a large current. 4) The bipolar transistor is liable for thermal runway(over heating) due to a negative temperature co-efficient. 5) BJTs are preferred for low current applications. 6) BJTs have low-medium input impedance(~1k -3k ohms). 7) BJTs are used where we need high gain& fast response. 8) BJT's have a higher cutoff frequency and a higher maximum current then FET's. 1) Once the gate terminal on an FET has been charged, no more current is needed to keep that transistor on (closed) for the duration of time required. 2) FETs are preferred in Wide line or load variations& have low power consumption. 3) FET function as voltage regulators as applied voltage on gate control the output characteristics. 4) FET have a positive temperature co- efficient, stopping thermal runway. 5) FETs are preferred in Low-voltage applications ( less than 250V). 6) no current flows through the gate, the input impedance of the FET is extremely large (in the range of 1010 ,1016 Ω). the large input impedance of the FET makes them an excellent choice for amplifier inputs. 7) FETs have low-medium gain. 8) FETs are preferred in High frequency applications ( greater than 200kHz).
  2. 2. Class No 09 Zunaib Ali 9) To operate BJTs at high switching frequencies & high current, we have to prevent the devices from going into haerd saturation as this will increase storage times( making it difficult to switch off quickly) but then cause the device to dissipate more power due to higher Vce-sat. 10) BJTs are relatively greater in size than FET of same rating. 11) BJT is temperature sensitive at higher level. 12) BJTs have high switching speed but they are noisy also. 13) BJT Have small duty cycles. 9) FETs are low switching devices.FET is therefore used for power switch design and high power functions( less than 500W output power). 10) FET are smaller in size.so area consumption of FET is less so Icsmade by FETs provide higher packing density as compared to BJTs. 11) FET is more stable to temperature & therefore it is used in high temperature applications. 12) As FET introduce lower noise level to the system so where sensitivity to the noise is very critcal and cannot be neglected,JFET amplifiers are preferred over BJT. 13) JFET used for micro wave communication such as VHF, UHF receivers. 14) FET's are easy to fabricate in large scale and have higher element density the BJT's. 15) JFET is mostly used in digital circuits. 16) Switch mode power supplies (SMPS): Hard switching above 200kHz 17) Switch mode power supplies (SMPS): ZVS below 1000 watts