Exam Flashcards
L-Section Adv and Disadv (2)
- Simplest Matching Technique
- Q is defined by the source and load impedence
Pi and T-section matching Adv & Disadv (4)
- Pi and T networks allows Q to be chosen by the designer to be ANY value GREATER than the loaded Q defined by the source and load impedence
- R_virt of Pi is always SMALLER than the source/load impedence
- T-network is used to match a LOW source/load impedence for a HIGH Q
- R_virt of T is always GREATER than source/load impedence
Wideband Matching Techniques and Limitation (2)
- Used to design a system which has a loaded Q which is LOWER than that possible with a simple L-section
- Transformers limit op. freq due to leakage inductance & winding cap
Configurations of Wideband matching techniques (2)
- Multiple L-sections, therefore lower Q value
- Impedence transformer
Reasons to use impedence transformer (2)
- When broader B/Ws are required than is possible with multiple L-sections
- Used when very high frequency required which is not possible with standard configuration transformers
L-Section Design Method (8)
- Find loaded Q (from source and load impedence)
- Find reactances of source and load (X_s and X_p)
- Capacitive reactance equivalent @ freq (for imag)
- Absorb stay cap into series cap
- Resonate out efffect of parrallel cap (find equiv reactance)
- Find required ind.
- Absorb inductor into mtching circuit
- Draw circuit
Methods of Biasing Small Signal Circuits (3)
- Constant Base Current Biasing
> Uses resistor R_b to provide appropriate base current
> Technique heavily dependent on beta (poorly defined for most transistors) - Shunt Feedback Biasing
> A measure of collector current is fed-back to the base reducing the overall effects of beta & V_BE spreads & temperature variations of beta - Potentiometer Biasing
> Uses potential divider with centre point connected to base
Method of biasing RF power amplifier (3)
- Forward biased diode connected in parallel with base
- Diode makes good thermal contact with the transistor to match temperature variations
- If temp rises, R_diode & voltage falls which reduces the base voltage of the transistor and shuts down to prevent thermal runaway
Where are Active biasing Networks used?
Used in circuits where temperature stability is of prime concern
Purpose of C_B and RFC (3)
- RFC provides an AC BLOCK for any RF signal that tries to flow into the supply
- C_B prevents any RF signal passing into supply or biasing network
- Both required to stop any undesired supply voltage variation & also any RF modulation of biasing network
Loaded Q (4)
- A measure of circuit quality or selectivity
- It provides a measure of circuit performance under loaded or real conditions
- Q is dependent on the source and load impedence
- Q is the circuit’s 3dB B/W
Effect of practical components on the performance of high freq circuits (3)
- Cap and inductors normally have resistance associated with them due to the need for:
> connecting leads
> inductor windings - Resistance can be translated into an equivalent parallel effective resistance
- Resistance appears in parallel with the source and load resistance which
> lowers the resistance value
> lowers loaded Q by equivalent amount
S and Y-parameter adv and disadv (3,2)
- S-parameters
> easy to measure (no ‘perfect’ RF short req.)
> less likely to destroy device under test during measurement of parameters
> can be used @ h.f relatively easily - Y-parameters
> difficult to measure @ h.f. due to obtaining R.F short
> short may destroy device
Definitions of: S_11 S_12 S_21 S_22
S_11 - Input reflection coeff
S_12 - reverse transmission coeff
S_21 - forward transmission coeff
S_22 - output reflection coeff
Actions taken for transistor instability at certain freq (5)
- Choose new transistor
- Determine values of source and load impedence
- Determine regions of stability on a smith chart
- Change the operating point (bias level) of the transistor to alter S-parameters and create new stable design
- Use a stabilisation network
Stub vs Lumped Element Matching Techniques Adv and Disadv (3)
- Stub matching attractive as freq increases because stub becomes more realistic in length
- As freq increases, lumped elements start to become less practical due to increasing parasitic effects
- Stub matching also has cost saving due to no L or C
Factors for picking substrate (5)
- Cost
- Dielectric constant
- Dissipation factor (tan (delta) - determines the insertion loss of the microstrip line)
- Ease of working / mechanical strength
- Temp stability
Methods of coupling resonant circuits - 1st (8)
- Capacitive Coupling
> Simple and lost-cost
> Cap C_12 determines the amount of coupling
> Frequency skew due to form of coupling
> 18dB/oct increase, 6dB/oct decrease
> Gradient difference due to effective presence of 3 reactive components (L-C-L) below resonance, only C above
> Cap too large, too much coupling and response broadens drastically = two resonant peaks
> Cap too small, not enough signal energy is passed from one resonant circuit to the other & insertion loss increases to unacceptable level
> C_12 = C/Q
Methods of coupling resonant circuits - 2nd (3)
- Inductive Coupling
> Directly analogous to capacitive coupling except freq response is in opposite direction.
> Design of transformer circuits is difficut & not very scientific. Coupling between coils depends on: ~ Coil Geometry ~ Coil Spacing ~ Cre material used ~ Degree of shielding
> L_12 = QL
Methods of coupling resonant circuits - 3rd (3)
- Active Coupling
> Use of transistors (FETs) to provide interstage coupling w/ high resolution between each stage
> Results in absence of freq response skews
> Signal only flows in 1 direction
> Increased cost & complexity
Courses of action taken for amplifier w/ specific gain required but not the MAG (2)
- Choose transistor w/required gain but is very time consuming
- Best method is to choose transistor with higher gain than req then selectively mismatch o/p by plotting a CG circle
Practical passive component construction and effects @ h.f (4)
- Devices w/ wire leads not commonly used @ r.f
- Modern surface mount devices chosen more
- These devices has extremely short leads, therefore less parasitic effects
- Need to be checked for resonance effects prior to use, to identify resonances that occur within bounds of particular design freq.
Design for Specified Gain (6)
- D_s
- D_2
- C_2
- Calculate G
- Centre of circle, r_0
- Radius of circle, p_0
Reflection co-efficient (2)
- Find distance to intersect of CG circle
- Find angle of intersection
