Transistor Bias Circuits [PDF]

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TRANSISTOR BIAS CIRCUITS& BJT AMPLIFIERS BIAS The DC Operating Point Voltage-Divider Bias Other Bias Methods A transistor must be properly biased with a dc voltage in order to operate as a linear amplifier. A dc operating point must be set so that signal variations at the input terminal are amplified and accurately reproduced at the output terminal. As you learned, when you bias a transistor, you establish the dc voltage and current values. This means, for example, that at the dc operating point, IC and VCE have specified values. The dc operating point is often referred to as the Q-point (quiescent point). Bias establishes the dc operating point (Q-point) for proper linear operation of an amplifier. If an amplifier is not biased with correct dc voltages on the input and output, it can go into saturation or cutoff when an input signal is applied



effects of proper and improper dc biasing Analisa



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The transistor in Figure 5–2(a) is biased with VCC and VBB to obtain certain values of IB, IC, IE, and VCE Ditentukan tiga(3) nilai IB untuk menganalisa yang terjadi pada IC dan VCE VBB diatur untuk mendapatkan nilai IB= 200µA. Karena IC = ßDC IB, maka akan didapatkan IC = 20mA. Untuk mendapatkan nilai VCE VCE = VCC - ICRC = 10 V - (20 mA)(220Ω) = 10 V - 4.4 V = 5.6 V



Q-point



Dengan cara yang sama



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Sehingga bisa digambarkan



Linear Operation The region along the load line including all points between saturation and cutoff is generally known as the linear region of the transistor’s operation. As long as the transistor is operated in this region, the output voltage is ideally a linear reproduction of the input Contoh transistor yang beroperasi pada daerah linear



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Dari circuit di atas tegangan sinusoidal Vin ditumpangkan pada VBB menyebabkan arus base bervariasi secara sinusoidal 100µA di bawah dan di atas Q-point yang bernilai 300µA. Hal ini juga menyebabkan nilai arus collector bervariasi 10 mA di bawah dan di atas Q-point 30mA. Sebagai akibat dari bervariasinya arus collector, tegangan collector to emitter juga bervariasi 2.2 V di bawah dan diatas Q-point 3.4 V. Titik A pada garis beban terhubung dengan puncak positif tegangan input sinusoidal, sedangkan titik B terhubung dengan dengan puncak negatif tegangan input sinusoidal. Titik Q terhubung dengan nilai nol (0) dari gelombang sinusoidal.



(1)VOLTAGE-DIVIDER BIAS You will now study a method of biasing a transistor for linear operation using a single source resistive voltage divider. This is the most widely used biasing method A more practical bias method is to use VCC as the single bias source, as shown below



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Contoh Tentukan nilai VCE dan IC untuk ßDC = 100 pada circuit berikut: Tegangan base:



Maka



(1)VOLTAGEDIVIDER BIAS (2)Emitter Bias (3)Base Bias (4)Emitter-Feedback Bias (5) CollectorFeedback Bias 5 | Page



BJT AMPLIFIERS (1) Amplifier Operation (2) Transistor AC Models (3) The Common-Emitter Amplifier (4) The Common-Collector Amplifier (5) The Common-Base Amplifier (6) Multistage Amplifiers (7) The Differential Amplifier (8) AC Quantities



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TRANSISTOR AC MODELS Comparison of the AC Beta ( ßac) to the DC Beta ( ßDC)



COMMON-EMITTER AMPLIFIER



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Phase Inversion



Ketika sinyal teg input berubah, maka Ib akan berubah. Ib berubah Ic juga berubah. Ib naik, Ic naik. Ic naik akan menyebabkan teg yg melalui Rc turun.



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AC Analysis To analyze the ac signal operation of an amplifier, an ac equivalent circuit is developed as follows: The capacitors C1, C2, and C3 are replaced by effective shorts because their values are selected so that XC is negligible at the signal frequency and can be considered to be 0 Ω The dc source is replaced by ground



Input Resistance at the Base Input resistnace pada base



Tegangan base



Karena Ie ≈ Ic



Subtitusi Vb dan Ib 9 | Page



maka



Output Resistance



Voltage Gain •



The gain is the ratio of ac output voltage at the collector (Vc) to ac input voltage at the base (Vb).



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Perhatikan gambar



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Maka diperoleh



•



Effect of the Emitter Bypass Capacitor on Voltage Gain (C2) Perhatikan gambar CE Amplifier The value of the bypass capacitor must be large enough so that its reactance over the frequency range of the amplifier is very small (ideally ) compared to RE. A good rule-of-thumb is that the capacitive reactance, XC, of the bypass capacitor should be at least 10 times smaller than RE at the minimum frequency for which the amplifier must operate.



Voltage Gain Without the Bypass Capacitor (C2) Without the bypass capacitor, the emitter is no longer at ac ground. Instead, RE is seen by the ac signal between the emitter and ground and effectively adds to re’ in the voltage gain formula



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Contoh Tentukan nilai minimum C2 pada rangkaian berikut jika amplifier harus beroperasi pada range frekuensi antara 200 Hz s.d 10 KHz Tentukan gain tegangan base-collector amplifier dengan dan tanpa C2 jika tdk terdapat resistor beban (load resistor) pada rangkaian re’ = 6.58Ω



Solusi



•



Tanpa C2, nilai gain adalah



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Dengan C2 nilai gain adalah



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•



Effect of a Load on the Voltage Gain



• • The collector resistance at the signal frequency is effectively RC in parallel with RL. Remember, the upper end of RC is effectively at ac ground The total ac collector resistance is Ganti RC dengan Rc pada voltage gain Contoh



Pada rangkaian di atas, jika resistor beban RL = 5 kΩ dihubungkan dengan output hitung voltage gain base-collector pada amplifier. Diketahui re’ = 6.58 Ω Solusi The ac collector resistance is Maka voltage gain adalah



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Current Gain



Power Gain



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